Forward GGSN originated DEACT PDP CTX REQ

This commit fixes TTCN3 sgsn test TC_attach_pdp_act_user_deact_mt.

Change-Id: I204209c017aac8a8402cbb8d0a0200540abcc954
diff --git a/include/osmocom/sgsn/gprs_sgsn.h b/include/osmocom/sgsn/gprs_sgsn.h
index 5f30889..8eba2d4 100644
--- a/include/osmocom/sgsn/gprs_sgsn.h
+++ b/include/osmocom/sgsn/gprs_sgsn.h
@@ -361,6 +361,7 @@
 struct sgsn_ggsn_ctx *sgsn_ggsn_ctx_by_id(uint32_t id);
 struct sgsn_ggsn_ctx *sgsn_ggsn_ctx_by_addr(struct in_addr *addr);
 struct sgsn_ggsn_ctx *sgsn_ggsn_ctx_find_alloc(uint32_t id);
+void sgsn_ggsn_ctx_drop_pdp(struct sgsn_pdp_ctx *pctx);
 int sgsn_ggsn_ctx_drop_all_pdp(struct sgsn_ggsn_ctx *ggsn);
 void sgsn_ggsn_ctx_add_pdp(struct sgsn_ggsn_ctx *ggc, struct sgsn_pdp_ctx *pdp);
 void sgsn_ggsn_ctx_remove_pdp(struct sgsn_ggsn_ctx *ggc, struct sgsn_pdp_ctx *pdp);