dtc/tests: don't include fdt.h prior to libfdt.h

tests will need fdt type definitions provided in a subsequent patch
to libfdt_env.h.  Since libfdt.h includes libfdt_env.h in the right
order anyway, just remove the fdt.h include.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>