selftests: gpio: update .gitignore

The executable that we build for GPIO selftests was renamed to
gpio-mockup-cdev. Let's update .gitignore so that we don't show it
as an untracked file.

Fixes: 8bc395a6 ("selftests: gpio: rework and simplify test implementation")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Kent Gibson <warthog618@gmail.com>