Mark game_mode_context_loop as 'noreturn'

The function contains the main-loop of the daemon and will indeed
never return.

daemon/dbus_messaging.h

@@ -38,7 +38,7 @@ POSSIBILITY OF SUCH DAMAGE.
 /**
  * Run the main D-BUS loop "forever"
  */
-void game_mode_context_loop(GameModeContext *context);
+void game_mode_context_loop(GameModeContext *context) __attribute__((noreturn));
 
 /**
  * Inhibit the screensaver