libgb: make sure all BSSGP functions have bssgp_ prefix

We change the minority of functions employing the gprs_bssgp_ prefix to
match with the majority without gprs_ in front.
diff --git a/openbsc/src/gprs/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index a45d344..a19af36 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -446,7 +446,7 @@
 		pinfo.ptmsi = &mm->p_tmsi;
 		pinfo.drx_params = mm->drx_parms;
 		pinfo.qos[0] = 0; // FIXME
-		rc = gprs_bssgp_tx_paging(mm->nsei, 0, &pinfo);
+		rc = bssgp_tx_paging(mm->nsei, 0, &pinfo);
 		rate_ctr_inc(&mm->ctrg->ctr[GMM_CTR_PAGING_PS]);
 		/* FIXME: queue the packet we received from GTP */
 		break;