virtio-mmio: Add explicit include for of.h

With linux/acpi.h no longer implicitly including of.h, add an explicit
include of of.h to fix the following error:

drivers/virtio/virtio_mmio.c:492:13: error: implicit declaration of function 'of_property_read_bool'; did you mean 'fwnode_property_read_bool'? [-Werror=implicit-function-declaration]

Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/virtio/virtio_mmio.c

···
 #include <linux/io.h>
 #include <linux/list.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/pm.h>
 #include <linux/slab.h>