doc: yat2m-stamp should depend on version.texi.

* doc/Makefile.am (yat2m-stamp): Depend on version.texi.

See the patch for libgpg-error:
https://lists.gnupg.org/pipermail/gnupg-devel/2018-April/033612.html

• Signed-off-by: Jernej Jakob <jernej.jakob@gmail.com>