util: Gate simple_mtx_assert_locked on !NDEBUG

..Instead of DEBUG so these work in debugoptimized builds.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17408>

src/util/simple_mtx.h

@@ -168,7 +168,7 @@ simple_mtx_unlock(simple_mtx_t *mtx)
 static inline void
 simple_mtx_assert_locked(simple_mtx_t *mtx)
 {
-#ifdef DEBUG
+#ifndef NDEBUG
    /* NOTE: this would not work for recursive mutexes, but
     * mtx_t doesn't support those
     */