misc: Put git-version-gen into the tarball

autoreconf does not work on released tarballs due the
missing git-version-gen. Add it.
diff --git a/Makefile.am b/Makefile.am
index e391de8..03bfdec 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -12,3 +12,5 @@
 	echo $(VERSION) > $@-t && mv $@-t $@
 dist-hook:
 	echo $(VERSION) > $(distdir)/.tarball-version
+
+EXTRA_DIST = git-version-gen