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 class='logheader'><td><span title='2021-06-05 17:24:13 +0200'>2021-06-05</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/algebra.scm?id=38eaba9a8e65ee286a5c6688e084fa618c329f82'>gnu: giac: Update to 1.7.0-11.</a></td><td>Nicolas Goaziou</td></tr>
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* gnu/packages/algebra.scm (giac): Update to 1.7.0-11.


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<tr class='logheader'><td><span title='2021-06-05 09:26:13 +0200'>2021-06-05</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/algebra.scm?id=c2b6d36b66c1c9ca5b1f7a5bf6416f1713cd76a7'>gnu: pari-gp: Use patched texlive-amsfonts.</a></td><td>Ricardo Wurmus</td></tr>
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* gnu/packages/algebra.scm (pari-gp)[native-inputs]: Use
texlive-amsfonts/patched.


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<tr class='logheader'><td><span title='2021-05-22 17:21:57 +0200'>2021-05-22</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/algebra.scm?id=4ea6852c5ff1606cf6848f3ddbb669120b228c13'>Merge branch 'master' into core-updates</a></td><td>Marius Bakke</td></tr>
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<tr class='logheader'><td><span title='2021-05-21 17:23:01 +0200'>2021-05-21</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/algebra.scm?id=f16162391309d1fec2a3f508edd14fbaf957470e'>gnu: giac: Update to 1.7.0-9.</a></td><td>Nicolas Goaziou</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/packages/algebra.scm (giac): Update to 1.7.0-9.


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