Made ground course from Xplane an unsigned int.

git-svn-id: https://arducopter.googlecode.com/svn/trunk@518 f9c3cf11-9bcb-44bc-f272-b75c42450872

libraries/AP_GPS/AP_GPS_IMU.cpp

@@ -182,7 +182,7 @@ void AP_GPS_IMU::join_data_xplane()
 	pitch_sensor = *(int *)&buffer[2];
 
 	 //Storing IMU heading (yaw)
-	ground_course = *(int *)&buffer[4];
+	ground_course = *(unsigned int *)&buffer[4];
 	
 	 //Storing airspeed
 	airspeed = *(int *)&buffer[6];