Avoid "multiple definition of `mystruct'" error from `ld`:

Submitted upstream at https://github.com/idris-lang/Idris-dev/issues/4914

--- a/test/ffi008/ffi008.h	1969-12-31 18:00:01.000000000 -0600
+++ a/test/ffi008/ffi008.h	2022-04-13 00:32:35.561262598 -0500
@@ -10,7 +10,7 @@
     int16_t b;
 };
 
-struct test2 mystruct;
+extern struct test2 mystruct;
 
 int size1(void);
 int size2(void);
--- a/test/ffi008/ffi008.c	1969-12-31 18:00:01.000000000 -0600
+++ a/test/ffi008/ffi008.c	2022-04-13 00:32:53.145186302 -0500
@@ -11,6 +11,8 @@
     return sizeof(struct test2);
 }
 
+struct test2 mystruct;
+
 void print_mystruct(void) {
     printf("a: %d b: %d\n", mystruct.a, mystruct.b);
 }
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<tr><td><span title='2022-04-29 18:07:16 +0200'>2022-04-29</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=9d7248cd87fbda8c717341a8092780782bef0820'>services: Add missing 'description' fields.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/databases.scm (postgresql-service-type)[description]: New field.
(memcached-service-type)[description]: New field.
(mysql-service-type)[description]: New field.
(redis-service-type)[description]: New field.
* gnu/services/desktop.scm (geoclue-service-type)[description]: New
field.
(udisks-service-type)[description]: New field.
(elogind-service-type)[description]: New field.
(account-service-type)[description]: New field.
* gnu/services/kerberos.scm (krb5-service-type)[description]: New field.
(pam-krb5-service-type)[description]: New field.
* gnu/services/lirc.scm (lirc-service-type)[description]: New field.
* gnu/services/mail.scm (dovecot-service-type)[description]: New field.
(opensmtpd-service-type)[description]: New field.
(mail-aliases-service-type)[description]: New field.
(exim-service-type)[description]: New field.
* gnu/services/monitoring.scm (zabbix-server-service-type)[description]:
New field.
(zabbix-agent-service-type)[description]: New field.
* gnu/services/nfs.scm (rpcbind-service-type)[description]: New field.
(pipefs-service-type)[description]: New field.
(gss-service-type)[description]: New field.
(idmap-service-type)[description]: New field.
* gnu/services/spice.scm (spice-vdagent-service-type)[description]: New field.
* gnu/services/sysctl.scm (sysctl-service-type)[description]: New field.
* gnu/services/virtualization.scm (libvirt-service-type)[description]:
New field.
(virtlog-service-type)[description]: New field.
* gnu/services/vpn.scm (openvpn-server-service-type)[description]: New field.
(openvpn-client-service-type)[description]: New field.
(wireguard-service-type)[description]: New field.
* gnu/services/web.scm (httpd-service-type)[description]: New field.
(fcgiwrap-service-type)[description]: New field.
(agate-service-type)[description]: New field.
[name]: Fix.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-31 15:36:49 +0200'>2021-03-31</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=4f547c50f9bc478fd7c8bb9d481e2e376b969e8a'>services: sysctl: Export &lt;sysctl-configuration&gt; record field accessors.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/sysctl.scm (sysctl-configuration-sysctl)
(sysctl-configuration-settings): Export.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>muradm</td></tr>
<tr><td><span title='2021-03-18 12:17:33 +0100'>2021-03-18</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=898489f48e436e45e86e1ba0fcdb6df5cd5a051a'>services: Enable "protected hardlinks" and "protected symlinks" by default.</a><span class='msg-avail'>...<span class='msg-tooltip'>References:

https://sysctl-explorer.net/fs/protected_hardlinks/
https://sysctl-explorer.net/fs/protected_symlinks/

* gnu/services/sysctl.scm (%default-sysctl-settings): New public variable.
(&lt;sysctl-configuration&gt;): Use %default-sysctl-settings as the default value.
* gnu/services/base.scm (%base-services): Add sysctl-service-type.
* doc/guix.texi (Miscellaneous Services): Document the new defaults.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Leo Famulari</td></tr>