selftests: add kselftest_install to .gitignore

Add kselftest_install directory to the .gitignore which is created while
creation of tar ball of objects:
make -C tools/testing/selftests gen_tar

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>