iOS: Fixed keyboard demo ignoring quit requests and window closing.

This also fixed a warning about an unused variable.

Xcode-iOS/Demos/src/keyboard.c

@@ -253,7 +253,7 @@ main(int argc, char *argv[])
 
     int done = 0;
     /* loop till we get SDL_Quit */
-    while (SDL_WaitEvent(&event)) {
+    while (!done && SDL_WaitEvent(&event)) {
         switch (event.type) {
         case SDL_QUIT:
             done = 1;