Add make dependency so that postgres.bki is rebuilt in major version change

Add make dependency so that postgres.bki is rebuilt in major version change

Every time since the current rule for postgres.bki was put in place
when we change the major version, people complain that their tests
fail in strange ways.  This is because the version number in
postgres.bki is not updated, because it has no dependency for that.
And you can't even force the rebuild manually if you don't happen to
know which file has the problem.  Fix that now before it will happen
again.

The only remaining problem with switching major versions, as far as
the regression tests are concerned, is that contrib needs to be
rebuilt.  But that's easily invoked, and in any case the failure modes
are more friendly if you forget that.