MAINTAINERS: Add selftests to DMA-BUF HEAPS FRAMEWORK entry

Include dmabuf-heaps selftests in the correct entry so that updates to it
can be sent to the right place.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240729081202.937-1-yuzenghui@huawei.com