AP_HAL_Linux: return RC protocol being decoded

libraries/AP_HAL_Linux/RCInput.h

@@ -27,6 +27,8 @@ public:
         return _rssi;
     }
 
+    const char *protocol() const override { return "Unknown"; }
+
     // default empty _timer_tick, this is overridden by board
     // specific implementations
     virtual void _timer_tick() {}