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_vty.c b/openbsc/src/libgb/gprs_bssgp_vty.c
index 49a90c3..aa1f106 100644
--- a/openbsc/src/libgb/gprs_bssgp_vty.c
+++ b/openbsc/src/libgb/gprs_bssgp_vty.c
@@ -167,7 +167,7 @@
 	return CMD_SUCCESS;
 }
 
-int gprs_bssgp_vty_init(void)
+int bssgp_vty_init(void)
 {
 	install_element_ve(&show_bssgp_cmd);
 	install_element_ve(&show_bssgp_stats_cmd);