Added license header to dtc/libfdt/fdt.h and libfdt_env.h

Added license header to dtc/libfdt/fdt.h and libfdt_env.h

This commit adds a license header to fdt.h and libfdt_env.h
because the license was omitted.

U-Boot note: the u-boot libfdt_env.h header portion was not applied to
the u-boot libfdt_env.h because that file was created by Gerald Van Baren
(with a license header). - gvb

Ref: DTC commit 27cdc1b1

Signed-off-by: Justin Sobota <jsobota@ti.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Gerald Van Baren <vanbaren@cideas.com>