contrib: fix makedistcheck with disabled systemd

EXTRA_DIST files need to be distributed, no matter if the systemd option
is configured or not.

Change-Id: Ibd348eac3adca88663bd510172efbe4ec0bf3599
diff --git a/contrib/systemd/Makefile.am b/contrib/systemd/Makefile.am
index e6ad299..fbb553f 100644
--- a/contrib/systemd/Makefile.am
+++ b/contrib/systemd/Makefile.am
@@ -1,5 +1,6 @@
-if HAVE_SYSTEMD
 EXTRA_DIST = osmo-ggsn.service
+
+if HAVE_SYSTEMD
 systemdsystemunit_DATA = \
   osmo-ggsn.service
 endif