wayland: Add a dummy function for the libdecor dismiss popup callback

This was never added, and we've been lucky that it's never called as libdecor doesn't check the function pointer for null before doing so.

src/video/wayland/SDL_waylandwindow.c

@@ -1435,10 +1435,16 @@ static void decoration_frame_commit(struct libdecor_frame *frame, void *user_dat
     }
 }
 
+static void decoration_dismiss_popup(struct libdecor_frame *frame, const char *seat_name, void *user_data)
+{
+    // NOP
+}
+
 static struct libdecor_frame_interface libdecor_frame_interface = {
     decoration_frame_configure,
     decoration_frame_close,
     decoration_frame_commit,
+    decoration_dismiss_popup
 };
 #endif