selftests/exec: add generated files to .gitignore

Add generated files non-regular and null-argv to .gitignore file.

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>