libfdt: remove unnecessary include directive from <linux/libfdt.h>

<linux/libfdt.h> is a wrapper of scripts/dtc/libfdt/libfdt.h
It should not do more than its job.  In fact, libfdt.h depends
on fdt.h, and scripts/dtc/libfdt/libfdt.h includes fdt.h already.
Trust the work in the upstream DTC project.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>