virtio-rng: 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/char/hw_random/virtio-rng.c

@@ -11,6 +11,7 @@
 #include <linux/virtio.h>
 #include <linux/virtio_rng.h>
 #include <linux/module.h>
+#include <linux/slab.h>
 
 static DEFINE_IDA(rng_index_ida);