Fix potential memory allocation mismatch from struct naming collision

Use anonymous namespace to prevent naming collision allocating
structs of the same name.

Should resolve #134419 although I was unable to redo this bug.

Ref !135726

source/blender/editors/space_image/image_buttons.cc

@@ -696,12 +696,17 @@ static void uiblock_layer_pass_buttons(uiLayout *layout,
   }
 }
 
+/* Prevent naming collision. */
+namespace {
+
 struct RNAUpdateCb {
   PointerRNA ptr = {};
   PropertyRNA *prop;
   ImageUser *iuser;
 };
 
+};  // namespace
+
 static void rna_update_cb(bContext *C, void *arg_cb, void * /*arg*/)
 {
   RNAUpdateCb *cb = (RNAUpdateCb *)arg_cb;