Use release helper from libosmocore

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

Change-Id: I3ccc8202b650268cc9d1721719ba2f205c351a67
Related: OS#1861
diff --git a/Makefile.am b/Makefile.am
index 447b148..67445ee 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -7,6 +7,8 @@
 
 EXTRA_DIST = asn1 .version README.md
 
+@RELMAKE@
+
 BUILT_SOURCES = $(top_srcdir)/.version
 $(top_srcdir)/.version:
 	echo $(VERSION) > $@-t && mv $@-t $@