rf_imp: fix race when stopping gain thread

detected with TSAN using benchmark_radio test

lib/src/phy/rf/rf_imp.c

@@ -175,7 +175,9 @@ int srsran_rf_close(srsran_rf_t* rf)
 {
   // Stop gain thread
   if (rf->thread_gain_run) {
+    pthread_mutex_lock(&rf->mutex);
     rf->thread_gain_run = false;
+    pthread_mutex_unlock(&rf->mutex);
     pthread_cond_signal(&rf->cond);
     pthread_join(rf->thread_gain, NULL);
   }

lib/src/radio/test/benchmark_radio.cc

@@ -25,6 +25,7 @@ extern "C" {
 //#undef I // Fix complex.h #define I nastiness when using C++
 #endif
 
+#include "srsran/common/tsan_options.h"
 #include "srsran/phy/utils/debug.h"
 #include "srsran/radio/radio.h"