virtio_input: pull in slab.h

In preparation to virtio header changes, include slab.h directly as
this module is using it.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_input.c

@@ -3,6 +3,7 @@
 #include <linux/virtio.h>
 #include <linux/virtio_config.h>
 #include <linux/input.h>
+#include <linux/slab.h>
 
 #include <uapi/linux/virtio_ids.h>
 #include <uapi/linux/virtio_input.h>