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/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 43ebbf8..75406c0 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -6,6 +6,7 @@
 bssgp_fc_init;
 bssgp_fc_ms_init;
 bssgp_msgb_alloc;
+bssgp_msgb_copy;
 bssgp_msgb_tlli_put;
 bssgp_parse_cell_id;
 bssgp_tx_bvc_block;