daemon: add game_mode_client_ref

Now that GameModeClient is reference counted, the counterpart to
game_mode_client_unref is also needed.

daemon/gamemode-context.c

@@ -638,6 +638,14 @@ void game_mode_client_unref(GameModeClient *client)
 	free(client);
 }
 
+void game_mode_client_ref(GameModeClient *client)
+{
+	if (!client) {
+		return;
+	}
+	atomic_fetch_add_explicit(&client->refcount, 1, memory_order_seq_cst);
+}
+
 /* Internal refresh config function (assumes no contention with reaper thread) */
 static void game_mode_reload_config_internal(GameModeContext *self)
 {

daemon/gamemode.h

@@ -54,6 +54,11 @@ typedef struct GameModeClient GameModeClient;
  */
 void game_mode_client_unref(GameModeClient *client);
 
+/**
+ * Increment the usage count of client.
+ */
+void game_mode_client_ref(GameModeClient *client);
+
 /**
  * Return the singleton instance
  */