Fix data race in enb::mac::ue class.

srsenb/hdr/stack/mac/ue.h

@@ -192,7 +192,7 @@ private:
   srsran::mac_pcap_net* pcap_net  = nullptr;
   uint64_t              conres_id = 0;
   uint16_t              rnti      = 0;
-  uint32_t              last_tti     = 0;
+  std::atomic<uint32_t> last_tti{0};
   uint32_t              nof_failures = 0;
 
   std::atomic<bool> active_state{true};