impl-node: make the group a string

We are currently using strings for the node group so actually store
this as a string or else all strings will end up in group 0.

src/pipewire/private.h

@@ -622,7 +622,7 @@ struct pw_impl_node {
 	char *name;				/** for debug */
 
 	uint32_t priority_driver;	/** priority for being driver */
-	uint32_t group_id;		/** group to schedule this node in */
+	char group[128];		/** group to schedule this node in */
 	uint64_t spa_flags;
 
 	unsigned int registered:1;