AP_HAL_SITL: Use double point precision when converting langitudes and longitudes from degrees to radians.

See #9536

libraries/AP_HAL_SITL/SITL_State.cpp

@@ -252,8 +252,8 @@ void SITL_State::_output_to_flightgear(void)
 
     fdm.version = 0x18;
     fdm.padding = 0;
-    fdm.longitude = radians(sfdm.longitude);
-    fdm.latitude = radians(sfdm.latitude);
+    fdm.longitude = DEG_TO_RAD_DOUBLE*sfdm.longitude;
+    fdm.latitude = DEG_TO_RAD_DOUBLE*sfdm.latitude;
     fdm.altitude = sfdm.altitude;
     fdm.agl = sfdm.altitude;
     fdm.phi   = radians(sfdm.rollDeg);