selftests: gpio: add gpio-cdev-uaf to .gitignore

selftests: gpio: add gpio-cdev-uaf to .gitignore

Commit c7f92042d3f3 ("selftests: gpio: Add gpio-cdev-uaf tests") added
the gpio-cdev-uaf binary to TEST_GEN_PROGS_EXTENDED but never added it
to .gitignore. Building it with:

    make -C tools/testing/selftests/gpio TARGETS=gpio

leaves gpio-cdev-uaf as an untracked file.

Fixes: c7f92042d3f3 ("selftests: gpio: Add gpio-cdev-uaf tests")
Signed-off-by: Cihan Karadag <cihan.cihan@gmail.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20260707235707.1349969-1-cihan.cihan@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>