Install systemd services with autotools

Change-Id: I563559f5b501eded44efafc60bb0c9ffdea20b3e
diff --git a/contrib/Makefile.am b/contrib/Makefile.am
new file mode 100644
index 0000000..3439c97
--- /dev/null
+++ b/contrib/Makefile.am
@@ -0,0 +1 @@
+SUBDIRS = systemd
diff --git a/contrib/systemd/Makefile.am b/contrib/systemd/Makefile.am
new file mode 100644
index 0000000..e6ad299
--- /dev/null
+++ b/contrib/systemd/Makefile.am
@@ -0,0 +1,5 @@
+if HAVE_SYSTEMD
+EXTRA_DIST = osmo-ggsn.service
+systemdsystemunit_DATA = \
+  osmo-ggsn.service
+endif
diff --git a/contrib/osmo-ggsn.service b/contrib/systemd/osmo-ggsn.service
similarity index 100%
rename from contrib/osmo-ggsn.service
rename to contrib/systemd/osmo-ggsn.service