MAINTAINERS: dmaengine: add header files directory

Somehow dmaengine header files are missed in the entry so update it

Link: https://lore.kernel.org/r/20210117070804.2539698-1-vkoul@kernel.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

MAINTAINERS

@@ -5281,6 +5281,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/vkoul/dmaengine.git
 F:	Documentation/devicetree/bindings/dma/
 F:	Documentation/driver-api/dmaengine/
 F:	drivers/dma/
+F:	include/linux/dma/
 F:	include/linux/dmaengine.h
 F:	include/linux/of_dma.h