build: include git-version-gen into the release tarball

Change-Id: I3d1c8c905f1acca6256eba51f6ee78214419e97a
diff --git a/Makefile.am b/Makefile.am
index 4b1006e..9f85801 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -22,7 +22,9 @@
              README.md \
              .version \
              debian \
-             contrib/libsmpp34.spec.in
+             contrib/libsmpp34.spec.in \
+             git-version-gen \
+             $(NULL)
 
 @RELMAKE@