noshutdown: Fix startup order

This rc script exists solely to create a file, so have it explicitly
require FILESYSTEMS. In its current form, it was as likely as not to
end up running before cleanvar, which would undo its work.

MFC after: 3 days
Fixes: 384d976725a5 ("rc.d: Add precious_machine rc.conf knob to create /var/run/noshutdown")
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D54119