Suppress FLEX_NO_BACKUP check for psqlscanslash.l.

Suppress FLEX_NO_BACKUP check for psqlscanslash.l.

The existing infrastructure for FLEX_NO_BACKUP doesn't work reliably
when two lexers are built in parallel in the same directory.  We can
probably fix that, but as a short-term workaround, just don't make
the check for psqlscanslash.l.

Per buildfarm.