util: Remove redundant defined(_WIN32) in u_string.h

This defined(_WIN32) is nested in an "#ifdef _WIN32",
so it's duplicated

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Acked-by: Jose Fonseca <jfonseca@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23897>

src/util/u_string.h

@@ -118,11 +118,11 @@ util_asprintf(char **str, const char *fmt, ...)
 
 #define strdup _strdup
 
-#if defined(_WIN32) && !defined(HAVE_STRTOK_R)
+#if !defined(HAVE_STRTOK_R)
 #define strtok_r strtok_s
 #endif
 
-#endif
+#endif /* _WIN32 */
 
 
 #ifdef __cplusplus