Drop gprs_msgb_copy with libosmocore replacement

gprs_msgb_copy was introduced in libosmocore 0.94
(f78ec5ce0d0f6038147d9b9e14d81094309ba5d5) as bssgp_msgb_copy. Let's use
that one to avoid code duplication.

Change-Id: I42a65fd8e4045fafadf5694f2d8d0c5e7ab350a0
diff --git a/tests/gbproxy/gbproxy_test.c b/tests/gbproxy/gbproxy_test.c
index 157da8b..a183320 100644
--- a/tests/gbproxy/gbproxy_test.c
+++ b/tests/gbproxy/gbproxy_test.c
@@ -1061,7 +1061,7 @@
 
 	if (received_messages) {
 		struct msgb *msg_copy;
-		msg_copy = gprs_msgb_copy(msg, "received_messages");
+		msg_copy = bssgp_msgb_copy(msg, "received_messages");
 		llist_add_tail(&msg_copy->list, received_messages);
 	}