daemon: make game_mode_client_unref public

So it can be used outside ouf gamemode-context.c.

daemon/gamemode.h

@@ -45,6 +45,15 @@ typedef struct GameModeContext GameModeContext;
 typedef struct GameModeConfig GameModeConfig;
 typedef struct GameModeClient GameModeClient;
 
+/**
+ * GameModeClient related functions
+ */
+
+/**
+ * Decrement the usage count of client.
+ */
+void game_mode_client_unref(GameModeClient *client);
+
 /**
  * Return the singleton instance
  */