Blimp: do not send MSG_RPM if RPM not enabled

Blimp/GCS_Mavlink.cpp

@@ -1,6 +1,7 @@
 #include "Blimp.h"
 
 #include "GCS_Mavlink.h"
+#include <AP_RPM/AP_RPM_config.h>
 
 MAV_TYPE GCS_Blimp::frame_type() const
 {
@@ -358,7 +359,9 @@ static const ap_message STREAM_EXTRA3_msgs[] = {
     MSG_MAG_CAL_PROGRESS,
     MSG_EKF_STATUS_REPORT,
     MSG_VIBRATION,
+#if AP_RPM_ENABLED
     MSG_RPM,
+#endif
     MSG_ESC_TELEMETRY,
     MSG_GENERATOR_STATUS,
 };