added rng_stuck to filter control status flags (#392)

EKF/common.h

@@ -433,6 +433,7 @@ union filter_control_status_u {
 		uint32_t mag_fault   : 1; ///< 18 - true when the magnetomer has been declared faulty and is no longer being used
 		uint32_t fuse_aspd   : 1; ///< 19 - true when airspeed measurements are being fused
 		uint32_t gnd_effect  : 1; ///< 20 - true when protection from ground effect induced static pressure rise is active
+		uint32_t rng_stuck   : 1; ///< 21 - true when rng data wasn't ready for more than 10s and new rng values haven't changed enough
 	} flags;
 	uint32_t value;
 };