depend-cleanup.sh: Fix overzealous abd_os.c cleanup

A source file with the same name is still used to build an abd_os.o, it
just comes from a different directory. Thus we need to include part of
the path in the regex too. Technically zfs/abd_os\.c would suffice given
it's now in sys/contrib/openzfs/lib/libzpool/abd_os.c, but this seems
clearer and less likely to have future false-positives.

Fixes: fd3d3240b0d7 ("depend-cleanup.sh: Add a rule to work around abd_os.c dependency changes")