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 <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

tools/spi/.gitignore

···
 # SPDX-License-Identifier: GPL-2.0-only
 spidev_fdx
 spidev_test
+include/