Linux and Google Play Services API fixes

PlatformEvent definition for Linux changed - forward decl of _XEvent instead of normal XEvent which required including X11/Xlib.h causing problems as described on the gameplay forums.

Also changed sample code integrating with Google Play Services, declaration of OnLeaderboardScoresLoadedListener.onLeaderboardScoresLoaded was changed ([API reference](http://developer.android.com/reference/com/google/android/gms/games/leaderboard/OnLeaderboardScoresLoadedListener.html))