Use release helper from libosmocore

Change-Id: I3b22a026ee61e88ed6d785182990cfcba0487bfe
Related: OS#1861
diff --git a/Makefile.am b/Makefile.am
index cb72faf..deb3299 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -9,3 +9,5 @@
 pkgconfig_DATA = libgtpnl.pc
 
 ${pkgconfig_DATA}: ${top_builddir}/config.status
+
+@RELMAKE@