move windows strtok_r define to u_string

This makes more sense for it, it's only used in the glsl compiler
currently, so we could probably move it there, but this seems fine for a
header only #define.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3024>

src/util/u_string.h

@@ -116,6 +116,10 @@ util_asprintf(char **str, const char *fmt, ...)
 
 #define strdup _strdup
 
+#if defined(_WIN32) && !defined(HAVE_STRTOK_R)
+#define strtok_r strtok_s
+#endif
+
 #endif