location-importer.in: add source column for overrides as well

location-importer.in: add source column for overrides as well

This allows us to track changes introduced by IP feeds from 3rd parties,
such as Amazon AWS, on the SQL server side.

In order not to break existing tables (which would required TRUNCATE),
there currently is no constraint set for the new column, but "NOT NULL"
is planned in the future.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>