configure.ac: distribute git-version-gen script
configure.ac: distribute git-version-gen script

git-version-gen must be included in a distribution in order to allow
running autoreconf on it.

Signed-off-by: Enrico Jorns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc