extres: syscon: pull in sys/malloc.h (fix !FDT)

syscon currently includes sys/malloc.h via header pollution from
dev/ofw/ofw_bus.h -> dev/ofw/openfirm.h. Fix the build without FDT
defined by including sys/malloc.h directly.

Reviewed by: andrew, imp, manu
Differential Revision: https://reviews.freebsd.org/D36787