Add default: break; to switches on SDL_Event::type

This is just a nice-to-have, as `SDL_Event::type` in an `Uint32`, so there are no
compiler errors about not handling all `SDL_EventType` enum values.

test/testrelative.c

@@ -47,6 +47,8 @@ static void loop(void)
             mouseX += event.motion.xrel;
             mouseY += event.motion.yrel;
         } break;
+        default:
+            break;
         }
     }
     for (i = 0; i < state->num_windows; ++i) {