build-sys: Include the .version file in the distribution tarball
As per the comments in git-version-gen, the .version file that is
typically generated automatically during build should be included
in the tarball, so generate it in the dist-hook.
This fixes a problem when running make distcheck which uses a read-only
source folder and thus cannot generate the $top_srcdir/.version as
part of it's build process (it uses a _build folder which is r/w)
and thus cannot be built.