selftests/dmabuf-heaps: add gitignore file

dmabuf-heaps builds a dmabuf-heap binary that can be ignored by git.
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/dmabuf-heaps/.gitignore

+dmabuf-heap