SITL: forward gimbal messages

A gimbal user needs to be able to receive gimbal device messages such as
GIMBAL_DEVICE_INFORMATION. Therefore, we need to forward this MAVLink
instance.

ROMFS/px4fmu_common/init.d-posix/px4-rc.mavlink

@@ -29,7 +29,7 @@ mavlink start -x -u $udp_offboard_port_local -r 4000000 -f -m onboard -o $udp_of
 mavlink start -x -u $udp_onboard_payload_port_local -r 4000 -f -m onboard -o $udp_onboard_payload_port_remote
 
 # Onboard link to gimbal
-mavlink start -x -u $udp_onboard_gimbal_port_local -r 400000 -m gimbal -o $udp_onboard_gimbal_port_remote
+mavlink start -x -u $udp_onboard_gimbal_port_local -r 400000 -f -m gimbal -o $udp_onboard_gimbal_port_remote
 
 # To display for SIH sitl
 if [ "$PX4_SIMULATOR" = "sihsim" ]; then