arm64: remove another unnecessary libfdt include path

Patch 63a4aea5 ("of: clean-up unnecessary libfdt include paths")
removed all explicit libfdt include paths, since those are no longer
necessary after the latest dtc upgrade. However, this one snuck in
during the same merge window. Remove it.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>