SGSN: Implement network-initiated PDP CTX DEACT when GGSN restarts

If the GGSN restarts, its restart counter will increase.  We can
detect that and accordingly release/delete all PDP contexts for
that GGSN.
diff --git a/openbsc/include/openbsc/gprs_gmm.h b/openbsc/include/openbsc/gprs_gmm.h
index b91b489..bd129ae 100644
--- a/openbsc/include/openbsc/gprs_gmm.h
+++ b/openbsc/include/openbsc/gprs_gmm.h
@@ -4,6 +4,7 @@
 #include <osmocore/msgb.h>
 #include <openbsc/gprs_sgsn.h>
 
+int gsm48_tx_gsm_deact_pdp_req(struct sgsn_pdp_ctx *pdp, uint8_t sm_cause);
 int gsm48_tx_gsm_act_pdp_rej(struct sgsn_mm_ctx *mm, uint8_t tid,
 			     uint8_t cause, uint8_t pco_len, uint8_t *pco_v);
 int gsm48_tx_gsm_act_pdp_acc(struct sgsn_pdp_ctx *pdp);