gb: Add bssgp_msgb_copy function

This function originates from openbsc/src/gprs but is just specific
to BSSGP/Gb on the same level like bssgp_msgb_alloc.

This commit puts the former gprs_msgb_copy function beside
bssgp_msgb_alloc.

Renamed function:

  gprs_msgb_copy -> bssgp_msgb_copy

Sponsored-by: On-Waves ehf
diff --git a/include/osmocom/gprs/gprs_bssgp.h b/include/osmocom/gprs/gprs_bssgp.h
index e24b563..c0b3f65 100644
--- a/include/osmocom/gprs/gprs_bssgp.h
+++ b/include/osmocom/gprs/gprs_bssgp.h
@@ -12,6 +12,7 @@
 /* gprs_bssgp_util.c */
 extern struct gprs_ns_inst *bssgp_nsi;
 struct msgb *bssgp_msgb_alloc(void);
+struct msgb *bssgp_msgb_copy(const struct msgb *msg, const char *name);
 const char *bssgp_cause_str(enum gprs_bssgp_cause cause);
 /* Transmit a simple response such as BLOCK/UNBLOCK/RESET ACK/NACK */
 int bssgp_tx_simple_bvci(uint8_t pdu_type, uint16_t nsei,