puzzles: allow using hints in Fifteen

This maps the select button to the "h" key for Fifteen only.

Change-Id: I7a5a61cec46e86254218fabfb191974f98c12319

apps/plugins/puzzles/rockbox.c

@@ -1493,7 +1493,10 @@ static int process_input(int tmo)
         break;
 
     case BTN_FIRE:
-        state = CURSOR_SELECT;
+        if(!strcmp("Fifteen", midend_which_game(me)->name))
+            state = 'h'; /* hint */
+        else
+            state = CURSOR_SELECT;
         break;
 
     default: