systemd-compat-units: Changed "After" line in run-postinsts.service.

In images containing the current oe-core master version of systemd,
remount-rootfs.service no longer exists. This service has been replaced by
systemd-remount-fs.service. By changing the "After" line in
run-postinsts.service, the script now succeeds on boot if enabled.

http://cgit.freedesktop.org/systemd/systemd/commit/?id=2b93b027d3a68b5d7ae26d0c2cd487eb5019d2a9

Signed-off-by: Matt Godshall <mgodshall@devonit.com>