spi: tools: Add include folder to .gitignore

spi: tools: Add include folder to .gitignore

The Makefile for the SPI tools creates an include/linux/spi folder and some
symlinks inside it. After running `make -C spi/tools`, this folder shows up
as untracked in the git status.
Add the above folder to the .gitignore file.

Fixes: f325b73dc4db ("spi: tools: move to tools buildsystem")
Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Link: https://patch.msgid.link/20260209095001.556495-1-flavra@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>