scripts/dtc: dtx_diff - update include dts paths to match build

Update the cpp include flags for compiling device tree dts files
to match the changes made to the kernel build process in
commit d5d332d3f7e8 ("devicetree: Move include prefixes from arch
to separate directory").

Cc: <stable@vger.kernel.org> # 4.12
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>