MAINTAINERS: Add myself as DMA-buf maintainer

As discussed on the list.
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/373539/

MAINTAINERS

@@ -5089,6 +5089,7 @@ F:	fs/dlm/
 
 DMA BUFFER SHARING FRAMEWORK
 M:	Sumit Semwal <sumit.semwal@linaro.org>
+M:	Christian König <christian.koenig@amd.com>
 L:	linux-media@vger.kernel.org
 L:	dri-devel@lists.freedesktop.org
 L:	linaro-mm-sig@lists.linaro.org (moderated for non-subscribers)