GPS/Arming: dual GPS difference check may need to ignore Z-axis difference

[rmackay9]

This report comes from this Copter-4.5 discussion where tomashmyh points out that when GPSx_TYPE = 14 (MAVLink) the driver ignores the altitude if GPS_INPUT_IGNORE_FLAG_ALT flag is set (see code here).

If the GPS can't provide altitude then we also shouldn't be using it in the Arming GPS consistency check.