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/libgb/gprs_bssgp_bss.c b/openbsc/src/libgb/gprs_bssgp_bss.c
index 4ca11ff..c058850 100644
--- a/openbsc/src/libgb/gprs_bssgp_bss.c
+++ b/openbsc/src/libgb/gprs_bssgp_bss.c
@@ -336,8 +336,8 @@
 }
 
 /* Parse a single GMM-PAGING.req to a given NSEI/NS-BVCI */
-int gprs_bssgp_rx_paging(struct bssgp_paging_info *pinfo,
-			 struct msgb *msg)
+int bssgp_rx_paging(struct bssgp_paging_info *pinfo,
+		    struct msgb *msg)
 {
 	struct bssgp_normal_hdr *bgph =
 			(struct bssgp_normal_hdr *) msgb_bssgph(msg);