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='2022-11-04 10:56:38 +0100'>2022-11-04</span></td><td><a href='/guix/commit/po/packages/tr.po?id=a65ce9dd500460af4f97e7398fb35dd817f4c677'>nls: Update translations.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
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<tr><td><span title='2022-07-09 21:49:34 +0200'>2022-07-09</span></td><td><a href='/guix/commit/po/packages/tr.po?id=e0d47d8facafb6d6c04741bbf30eefbc68aaefb9'>nls: Update translations.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
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