geofence: lat/lon is double

This is the same change as 5832948371866aec8f0c7f16b13869f270d36aad but
against the master branch.

Some time ago the type of lat and lon in the global_pos uorb topic was
changed but this use here was missed.
Merge pull request #1366 from PX4/geofencehotfix

geofence: lat/lon is double