gtp: add pdp_set_imsi_nsapi

Encapsulate code to handle the teid with GTPv0.
diff --git a/gtp/pdp.h b/gtp/pdp.h
index b069a6f..6e30467 100644
--- a/gtp/pdp.h
+++ b/gtp/pdp.h
@@ -242,6 +242,8 @@
 int pdp_tiddel(struct pdp_t *pdp);
 int pdp_tidget(struct pdp_t **pdp, uint64_t tid);
 
+void pdp_set_imsi_nsapi(struct pdp_t *pdp, uint64_t teid);
+
 /*
 int pdp_iphash(void* ipif, struct ul66_t *eua);
 int pdp_ipset(struct pdp_t *pdp, void* ipif, struct ul66_t *eua);