Properly split llc->sndcp->gtp unitdata pathi through extra func call

Beforehand the function handling the LL-UNITDATA primitive from LLC was
already submitting the packet to GTP code which had an SNDCP related
name, so everything was really confusing and layer separation was not
clean.

Change-Id: Ia544a9dd4c0c7647b0c1b64ca110351f40820618
diff --git a/include/osmocom/sgsn/gtp.h b/include/osmocom/sgsn/gtp.h
index 85a50d3..e835b53 100644
--- a/include/osmocom/sgsn/gtp.h
+++ b/include/osmocom/sgsn/gtp.h
@@ -3,3 +3,6 @@
 struct sgsn_instance;
 
 int sgsn_gtp_init(struct sgsn_instance *sgi);
+
+int sgsn_gtp_data_req(struct gprs_ra_id *ra_id, int32_t tlli, uint8_t nsapi,
+		      struct msgb *msg, uint32_t npdu_len, uint8_t *npdu);