Ensure guc_tables.o's dependency on guc_tables.inc.c is known.

Ensure guc_tables.o's dependency on guc_tables.inc.c is known.

Without this, rebuilds can malfunction unless --enable-depend is used.
Historically we've expected that you can get away without
--enable-depend as long as you manually clean after changing *.h
files; the makefiles are supposed to handle other sorts of
dependencies.  So add this one.

Follow-on to 635998965, so no need for back-patch.

Discussion: https://postgr.es/m/3121329.1758650878@sss.pgh.pa.us