Change obmc-standby to multi-user

Change obmc-standby.target to multi-user.target for post code manager service.

Tested:
Post code could be catured correctly.

Change-Id: I3686104f67aad13fa4d5f5d382d3eb7dd8b9a74f
Signed-off-by: Kuiying Wang <kuiying.wang@intel.com>
diff --git a/service_files/xyz.openbmc_project.State.Boot.PostCode.service b/service_files/xyz.openbmc_project.State.Boot.PostCode.service
index 67bc43f..4246472 100644
--- a/service_files/xyz.openbmc_project.State.Boot.PostCode.service
+++ b/service_files/xyz.openbmc_project.State.Boot.PostCode.service
@@ -8,4 +8,4 @@
 BusName=xyz.openbmc_project.State.Boot.PostCode
 
 [Install]
-WantedBy=obmc-standby.target
+WantedBy=multi-user.target