ue,rrc_nr: mark is_security_active as true, after updating SK counter.

srsue/src/stack/rrc_nr/rrc_nr.cc

@@ -1021,6 +1021,7 @@ bool rrc_nr::configure_sk_counter(uint16_t sk_counter)
   if (usim->generate_nr_context(sk_counter, &sec_cfg) == false) {
     return false;
   }
+  security_is_activated = true;
   return true;
 }