audio/avrcp: Assing ids when registering a player

This gives the local player an id so when respoding to GetFolderItem they
are distinguishable.
diff --git a/profiles/audio/avrcp.c b/profiles/audio/avrcp.c
index 29fd67a..750bdd9 100644
--- a/profiles/audio/avrcp.c
+++ b/profiles/audio/avrcp.c
@@ -4058,12 +4058,14 @@
 	struct avrcp_server *server;
 	struct avrcp_player *player;
 	GSList *l;
+	static uint16_t id = 0;
 
 	server = find_server(servers, adapter);
 	if (!server)
 		return NULL;
 
 	player = g_new0(struct avrcp_player, 1);
+	player->id = ++id;
 	player->server = server;
 	player->cb = cb;
 	player->user_data = user_data;