build: Make sure version.texi is generated in time.

PGP Signed Part:Good signature from AA4373E6C82C78AD Damien Goutte-Gattat <dgouttegattat@incenp.org> (trust undefined) created at 2018-04-11T17:05:27+0200 using RSA
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• doc/Makefile.am (yat2m-stamp): Depend on version.texi. --

When building from a cloned Git repository and with make -j 3
(or higher), the version.texi file may not have been generated
yet when yat2m is called to generate the man page, resulting in
a build failure.

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>