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/src/gprs/gprs_gmm.c b/src/gprs/gprs_gmm.c
index 91f769d..bea63dc 100644
--- a/src/gprs/gprs_gmm.c
+++ b/src/gprs/gprs_gmm.c
@@ -2648,7 +2648,7 @@
 	 * and the dynamic resolution will be the right thing
 	 * in the long run.
 	 */
-	msg = gprs_msgb_copy(_msg, __func__);
+	msg = bssgp_msgb_copy(_msg, __func__);
 	if (!msg) {
 		struct gsm48_hdr *gh = (struct gsm48_hdr *) msgb_gmmh(_msg);
 		uint8_t transaction_id = gsm48_hdr_trans_id(gh);