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

MAINTAINERS

@@ -6530,6 +6530,7 @@ F:	drivers/dma-buf/dma-heap.c
 F:	drivers/dma-buf/heaps/*
 F:	include/linux/dma-heap.h
 F:	include/uapi/linux/dma-heap.h
+F:	tools/testing/selftests/dmabuf-heaps/
 
 DMC FREQUENCY DRIVER FOR SAMSUNG EXYNOS5422
 M:	Lukasz Luba <lukasz.luba@arm.com>