Cleanup: Strict compiler warning in GPU code

The NDEBUG is a toggle define, and in debug builds it is not defined.

This change solves the warning
  mtl_context.mm:2282:5: warning: 'NDEBUG' is not defined, evaluates to 0 [-Wundef]

Pull Request: https://projects.blender.org/blender/blender/pulls/112504

source/blender/gpu/metal/mtl_context.mm

@@ -39,7 +39,11 @@ using namespace blender::gpu;
 
 /* Fire off a single dispatch per encoder. Can make debugging view clearer for texture resources
  * associated with each dispatch. */
-#define MTL_DEBUG_SINGLE_DISPATCH_PER_ENCODER 1 && !NDEBUG
+#if defined(NDEBUG)
+#  define MTL_DEBUG_SINGLE_DISPATCH_PER_ENCODER 0
+#else
+#  define MTL_DEBUG_SINGLE_DISPATCH_PER_ENCODER 1
+#endif
 
 /* Debug option to bind null buffer for missing UBOs.
  * Enabled by default. TODO: Ensure all required UBO bindings are present. */