SDL_x11modes: fix -Wunused-variable

src/video/x11/SDL_x11modes.c

@@ -833,6 +833,8 @@ freeInfo:
             return SDL_SetError("X11_XRRSetCrtcConfig failed");
         }
     }
+#else
+    (void)data;
 #endif /* SDL_VIDEO_DRIVER_X11_XRANDR */
 
     return 0;