[ahrs] int_cmpl_quat: configurable AHRS_HEADING_UPDATE_GPS_MIN_SPEED

Only used if AHRS_USE_GPS_HEADING is true.
Don't update heading from GPS course if GPS ground speed is below is this threshold.
Defaults to 5.0m/s which is the same as hardcoded value before.
[conf] lower GUIDANCE_H IGAIN on some quadrotor airframes