systemd-units: ensure required class is inherited

Although the project is trying to get away from the use of
obmc-phosphor-systemd, need to ensure it's being used where required.
This recipe utilizes it to create chassis/host synchronization and
action targets.

Change-Id: I90427eaa5fa66269455fb96b7ed5f06ae9f6180a
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/recipes-phosphor/state/phosphor-state-manager_git.bb b/recipes-phosphor/state/phosphor-state-manager_git.bb
index a73f387..5d2357e 100644
--- a/recipes-phosphor/state/phosphor-state-manager_git.bb
+++ b/recipes-phosphor/state/phosphor-state-manager_git.bb
@@ -44,6 +44,7 @@
 
 inherit meson pkgconfig
 inherit obmc-phosphor-dbus-service
+inherit obmc-phosphor-systemd
 
 DEPENDS += "sdbusplus"
 DEPENDS += "sdeventplus"