MAINTAINERS: Add related headers to IOMMU section

The linux/iommu.h and linux/iova.h headers belong to the IOMMU subsystem
but scripts/get_maintainers.pl currently fails to assign them because
they aren't listed in MAINTAINERS.
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

MAINTAINERS

@@ -6764,6 +6764,8 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git
 S:	Maintained
 F:	Documentation/devicetree/bindings/iommu/
 F:	drivers/iommu/
+F:	include/linux/iommu.h
+F:	include/linux/iova.h
 
 IP MASQUERADING
 M:	Juanjo Ciarlante <jjciarla@raiz.uncu.edu.ar>