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='2017-05-04 22:45:53 +0300'>2017-05-04</span></td><td><a href='/guix/commit/etc/indent-code.el.in?id=97658183eda6e196bd16bed02838d321b45aee70'>etc: indent-code.el: Define source tree by "indent-code.el".</a><span class='msg-avail'>...</span></td><td>Alex Kost</td></tr>
<tr><td><span title='2017-01-20 21:39:17 +0100'>2017-01-20</span></td><td><a href='/guix/commit/etc/indent-code.el.in?id=830d4c1fcdff7be199e66cd70a5048264b54a151'>etc: indent-code.el: Add indentation info for non-Guix Scheme constructs.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-13 18:49:31 +0100'>2017-01-13</span></td><td><a href='/guix/commit/etc/indent-code.el.in?id=557d9c8d7a843bf06e75b4e1a742654ccf951fa3'>etc: Support indentation of whole files.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>