compat: fix static_assert

It was not possible to use it outside of functions.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

include/vlc_fixups.h

@@ -273,7 +273,9 @@ static inline locale_t newlocale(int mask, const char * locale, locale_t base)
 #endif
 
 #if !defined (HAVE_STATIC_ASSERT) && !defined(__cpp_static_assert)
-# define _Static_assert(x, s) ((void) sizeof (struct { unsigned:-!(x); }))
+# define STATIC_ASSERT_CONCAT_(a, b) a##b
+# define STATIC_ASSERT_CONCAT(a, b) STATIC_ASSERT_CONCAT_(a, b)
+# define _Static_assert(x, s) extern char STATIC_ASSERT_CONCAT(static_assert_, __LINE__)[sizeof(struct { unsigned:-!(x); })]
 # define static_assert _Static_assert
 #endif