tools: Include U-Boot libfdt headers from their actual path

There are no headers for libfdt in lib/libfdt, as they are instead
located in scripts/dtc/libfdt. Specifying lib/libfdt for headers
inclusion in host tools results in using the system libfdt headers,
which is not what we want. Change this to the proper path.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>