Add systemd unit file

Refs: #4594
Change-Id: I80fea25ed4a612cf015c5344d26f2ee8875a3680
diff --git a/wscript b/wscript
index 2dd61e4..2a5e4c3 100644
--- a/wscript
+++ b/wscript
@@ -69,3 +69,9 @@
     bld.recurse('examples')
 
     bld.install_files('${SYSCONFDIR}/ndn', 'repo-ng.conf.sample')
+
+    if Utils.unversioned_sys_platform() == 'linux':
+        bld(features='subst',
+            name='repo-ng.service',
+            source='systemd/repo-ng.service.in',
+            target='systemd/repo-ng.service')