Install systemd services with autotools

Change-Id: I90f1efc4433ec641a28931d5c19e2301a67cf1e9
diff --git a/contrib/systemd/Makefile.am b/contrib/systemd/Makefile.am
new file mode 100644
index 0000000..507be2d
--- /dev/null
+++ b/contrib/systemd/Makefile.am
@@ -0,0 +1,5 @@
+if HAVE_SYSTEMD
+EXTRA_DIST = osmo-hnbgw.service
+systemdsystemunit_DATA = \
+  osmo-hnbgw.service
+endif