ekf2: GNSS yaw, use reported yaw accuracy when available

src/modules/ekf2/EKF/common.h

@@ -155,6 +155,7 @@ struct gpsMessage {
 	int32_t     alt{};              ///< Altitude in 1E-3 meters (millimeters) above MSL
 	float       yaw{};              ///< yaw angle. NaN if not set (used for dual antenna GPS), (rad, [-PI, PI])
 	float       yaw_offset{};       ///< Heading/Yaw offset for dual antenna GPS - refer to description for GPS_YAW_OFFSET
+	float       yaw_accuracy{};	///< yaw measurement accuracy (rad, [0, 2PI])
 	uint8_t     fix_type{};         ///< 0-1: no fix, 2: 2D fix, 3: 3D fix, 4: RTCM code differential, 5: Real-Time Kinematic
 	float       eph{};              ///< GPS horizontal position accuracy in m
 	float       epv{};              ///< GPS vertical position accuracy in m
@@ -198,6 +199,7 @@ struct gpsSample {
 	float       hacc{};     ///< 1-std horizontal position error (m)
 	float       vacc{};     ///< 1-std vertical position error (m)
 	float       sacc{};     ///< 1-std speed error (m/sec)
+	float       yaw_acc{};  ///< 1-std yaw error (rad)
 };
 
 struct magSample {