Incrementally running 'make syntax-check' on a tree previously
built after commit 62dee6fa but before 44036460 fails sc_po_check
(because the generated qemu_dispatch.h gained translatable strings).
This is a followup to commit addaa537 for that scenario.

* cfg.mk (sc_po_check): Add another prereq.
($(srcdir)/daemon/qemu_dispatch.h): Add rule.