Use release helper from libosmocore

See
https://osmocom.org/projects/cellular-infrastructure/wiki/Make_a_new_release
for details.

Change-Id: Iafe5905895a656b7055d58ca8f745fcfa4bd9886
Related: OS#1861
diff --git a/Makefile.am b/Makefile.am
index 798c092..6caa009 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -4,6 +4,9 @@
 
 BUILT_SOURCES = $(top_srcdir)/.version
 EXTRA_DIST = git-version-gen .version
+
+@RELMAKE@
+
 $(top_srcdir)/.version:
 	echo $(VERSION) > $@-t && mv $@-t $@
 dist-hook: