Makefile.am: add 'git-version-gen' to EXTRA_DIST

Change-Id: Idbf4bf70d83fbf697d4a094dbb47592deaea6114
diff --git a/Makefile.am b/Makefile.am
index 1f51db9..69ce8ff 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -8,6 +8,7 @@
 	     README.md \
 	     contrib/osmo-pcu.spec.in \
 	     debian \
+	     git-version-gen \
 	     osmoappdesc.py \
 	     $(NULL)