xfstests: fix depend targets

xfstests: fix depend targets

There's no need to re-make the dependency files all the time.  Make
it so the "depend" target rebuilds the ".dep" file only if necessary.
Also change the name of the dependency file created for "ltdepend"
to be ".ltdep".

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>