Fix data race in is_first_ul_grant.

srsue/hdr/stack/mac/mac.h

@@ -150,7 +150,7 @@ private:
 
   // pointer to MAC PCAP object
   srsran::mac_pcap* pcap = nullptr;
-  bool              is_first_ul_grant = false;
+  std::atomic<bool> is_first_ul_grant{false};
 
   std::mutex    metrics_mutex                = {};
   mac_metrics_t metrics[SRSRAN_MAX_CARRIERS] = {};