systemd: Own service startup ordering with respect to pldmd

We're about to turn the dependencies around so mctp-demux-daemon can be
socket-activated by systemd. Move ownership of the start-up sequencing
between mctp-demux-daemon and pldmd into libmctp so the switch to
socket-activation is transparent to pldmd.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: Ia094d7ce32eabccf86d6a53a4a74b7deea79488d
diff --git a/systemd/system/mctp-demux.service b/systemd/system/mctp-demux.service
index ad09b30..c13e871 100644
--- a/systemd/system/mctp-demux.service
+++ b/systemd/system/mctp-demux.service
@@ -1,5 +1,7 @@
 [Unit]
 Description=MCTP demultiplexer daemon
+Wants=pldmd.service
+Before=pldmd.service
 
 [Service]
 Restart=always