libgb: remove dependencies to openbsc/vty.h and openbsc/gsm_data.h

Rather than using openbsc internal data/functions, we now use only
internal and libosmocore-provided ones.
diff --git a/openbsc/src/gprs/sgsn_main.c b/openbsc/src/gprs/sgsn_main.c
index 9600c23..98cc600 100644
--- a/openbsc/src/gprs/sgsn_main.c
+++ b/openbsc/src/gprs/sgsn_main.c
@@ -226,7 +226,7 @@
 	if (rc < 0)
 		exit(1);
 
-	sgsn_nsi = gprs_ns_instantiate(&sgsn_ns_cb);
+	sgsn_nsi = gprs_ns_instantiate(&sgsn_ns_cb, tall_bsc_ctx);
 	if (!sgsn_nsi) {
 		LOGP(DGPRS, LOGL_ERROR, "Unable to instantiate NS\n");
 		exit(1);