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