gprs_ns2: add gprs_ns2_free_nses() to free all NS-E

Allow users to clean up most of the state (e.g. the PCU).

Change-Id: Ia00753a64b7622a0864341f51ea49b6963543755
diff --git a/include/osmocom/gprs/gprs_ns2.h b/include/osmocom/gprs/gprs_ns2.h
index 2c58723..7531530 100644
--- a/include/osmocom/gprs/gprs_ns2.h
+++ b/include/osmocom/gprs/gprs_ns2.h
@@ -101,6 +101,7 @@
 struct gprs_ns2_nse *gprs_ns2_create_nse(struct gprs_ns2_inst *nsi, uint16_t nsei);
 uint16_t gprs_ns2_nse_nsei(struct gprs_ns2_nse *nse);
 void gprs_ns2_free_nse(struct gprs_ns2_nse *nse);
+void gprs_ns2_free_nses(struct gprs_ns2_inst *nsi);
 
 /* create vc */
 void gprs_ns2_free_nsvc(struct gprs_ns2_vc *nsvc);