Use osmocom-style git-version-gen / .version magic

This generates the version number from the latest tag + git history

Change-Id: I71e1c3f8f534d9e9690c94068eccdee29a6fee12
diff --git a/debian/rules b/debian/rules
index 5562909..8710fa0 100755
--- a/debian/rules
+++ b/debian/rules
@@ -12,3 +12,7 @@
 
 override_dh_strip:
 	dh_strip --dbg-package=openggsn-dbg
+
+override_dh_autoreconf:
+	echo $(VERSION) > .tarball-version
+	dh_autoreconf