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: I5014d98e06e033d04be45585b34458c90b1ced00
diff --git a/contrib/systemd/Makefile.am b/contrib/systemd/Makefile.am
index c0d95ff..340cd90 100644
--- a/contrib/systemd/Makefile.am
+++ b/contrib/systemd/Makefile.am
@@ -1,5 +1,6 @@
-if HAVE_SYSTEMD
 EXTRA_DIST = osmo-mgw.service
+
+if HAVE_SYSTEMD
 systemdsystemunit_DATA = \
   osmo-mgw.service
 endif