oneshot: discover-system-state: remain after exit

This service should only ever run once, after a BMC has first booted.
Ensure this is the case by setting RemainAfterExit to yes.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I346ab3588ee210709206a41c5e2acc73222561eb
diff --git a/service_files/phosphor-discover-system-state@.service b/service_files/phosphor-discover-system-state@.service
index 21554c9..6b966f0 100644
--- a/service_files/phosphor-discover-system-state@.service
+++ b/service_files/phosphor-discover-system-state@.service
@@ -12,6 +12,7 @@
 [Service]
 Restart=no
 Type=oneshot
+RemainAfterExit=yes
 ExecStart=/usr/bin/phosphor-discover-system-state --host %i
 
 [Install]