virtio_features: make it self-contained

virtio_features.h uses WARN_ON_ONCE and memset so it must
include linux/bug.h and linux/string.h

Message-ID: <579986aa9b8d023844990d2a0e267382f8ad85d5.1764873799.git.mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: mstsirkin

include/linux/virtio_features.h

@@ -3,6 +3,8 @@
 #define _LINUX_VIRTIO_FEATURES_H
 
 #include <linux/bits.h>
+#include <linux/bug.h>
+#include <linux/string.h>
 
 #define VIRTIO_FEATURES_U64S	2
 #define VIRTIO_FEATURES_BITS	(VIRTIO_FEATURES_U64S * 64)