Fix missing extern C for __asan_default_options

(fix of commit 84b695d)

Issue #206 (bdwgc).

* os_dep.c [ADDRESS_SANITIZER && (UNIX_LIKE || NEED_FIND_LIMIT
|| MPROTECT_VDB) && !CUSTOM_ASAN_DEF_OPTIONS] (__asan_default_options):
Add the declaration wrapped into EXTERN_C_BEGIN.

os_dep.c

@@ -515,6 +515,10 @@ GC_INNER const char * GC_get_maps(void)
 #if defined(ADDRESS_SANITIZER) && (defined(UNIX_LIKE) \
                     || defined(NEED_FIND_LIMIT) || defined(MPROTECT_VDB)) \
     && !defined(CUSTOM_ASAN_DEF_OPTIONS)
+  EXTERN_C_BEGIN
+  GC_API const char *__asan_default_options(void);
+  EXTERN_C_END
+
   /* To tell ASan to allow GC to use its own SIGBUS/SEGV handlers.      */
   /* The function is exported just to be visible to ASan library.       */
   GC_API const char *__asan_default_options(void)