Add GDBvn.texi and version.subst to the release tar file,
so the gdb.info does not need makeinfo.

This avoids the need for makeinfo to be available.

2020-12-04  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* Makefile.in: Delete GDBvn.texi and version.subst only in
	the maintainer-clean target.