build: provide git-head revision via githead.h

build: provide git-head revision via githead.h

We need the git-revision for module-version checks so provide the
infrastructure now and print it in log_init().

Note that the git-describe string is distributed with the tarballs so
end-users will not have to generate it themself. But when building from
git, the revision will be automatically updated whenever something
changes.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>