Make fifo.c thread safe

include/hamlib/rig.h

@@ -137,6 +137,11 @@ typedef struct
     int head;
     int tail;
     int flush;  // flush flag for stop_morse
+#if _PTHREAD_H
+    pthread_mutex_t mutex;
+#else
+    int mutex;
+#endif
 } FIFO_RIG;