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

Change-Id: I4100459461c04e57cbdfa072dcf1c771f0c0b798
diff --git a/Makefile.am b/Makefile.am
index d55eace..22ecc50 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -17,6 +17,7 @@
 	     README.md \
 	     contrib/libosmo-abis.spec.in \
 	     debian \
+	     git-version-gen \
 	     $(NULL)
 
 @RELMAKE@