tools: gpio: Add include/linux/gpio.h to .gitignore

File include/linux/gpio.h is generated after building
tools/testing/selftests

Add gpio.h to .gitignore to help clean up working tree status.

Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>