ostree-prepare-root.service: Also order before plymouth-switch-root.service

In the OSTree model, /sysroot gets set up twice.  We need to ensure
that the /sysroot plymouth sees is only after OSTree has set it up.

src/boot/ostree-prepare-root.service

@@ -23,6 +23,7 @@ ConditionPathExists=/etc/initrd-release
 OnFailure=emergency.service
 After=initrd-switch-root.target
 Before=initrd-switch-root.service
+Before=plymouth-switch-root.service
 
 [Service]
 Type=oneshot