GCS_MAVLink: correct static assert for stream rate counts

C++-17 warning under clang

Author: peterbarker

libraries/GCS_MAVLink/GCS_MAVLink_Parameters.cpp

@@ -96,7 +96,7 @@ static const uint8_t default_rates[] {
     AP_MAV_DEFAULT_STREAM_RATE_PARAMS,
     AP_MAV_DEFAULT_STREAM_RATE_ADSB,
 };
-static_assert(ARRAY_SIZE(default_rates) == GCS_MAVLINK::NUM_STREAMS);
+static_assert(ARRAY_SIZE(default_rates) == GCS_MAVLINK::NUM_STREAMS, "need same default rates as NUM_STREAMS");
 
 #define DRATE(x) (float(default_rates[x]))