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: I4b1973a4796ccddce930a130be00c15b09572c16
diff --git a/contrib/systemd/Makefile.am b/contrib/systemd/Makefile.am
index b644f34..11e8f98 100644
--- a/contrib/systemd/Makefile.am
+++ b/contrib/systemd/Makefile.am
@@ -1,9 +1,13 @@
+EXTRA_DIST = \
+  osmo-gbproxy.service \
+  osmo-gtphub.service \
+  osmo-sgsn.service
+
 if HAVE_SYSTEMD
 SYSTEMD_SERVICES = \
   osmo-gbproxy.service \
   osmo-gtphub.service \
   osmo-sgsn.service
 
-EXTRA_DIST = $(SYSTEMD_SERVICES)
 systemdsystemunit_DATA = $(SYSTEMD_SERVICES)
 endif