stand/fdt: Swap libfdt include order

libfdt.h should be included before fdt.h, as hinted at by all of libfdt/;
standard include order being libfdt.h, libfdt_env.h, fdt.h.

The current include order also causes problems when libfdt gets updated, as
fdt.h requires some definitions from libfdt.h.

Differential Revision: https://reviews.freebsd.org/D13688