proc_ra: fix race detected with TSAN

rar_received is accessed from PHY worker and Stack thread

srsue/hdr/stack/mac/proc_ra.h

@@ -148,7 +148,7 @@ private:
 
   bool              started_by_pdcch = false;
   uint32_t          rar_grant_nbytes = 0;
-  bool     rar_received     = false;
+  std::atomic<bool> rar_received     = {false};
 };
 
 } // namespace srsue