virtio: Move extern declaration to header file

This fixes a warning from smatch (static code analyser).
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/virtio/vhost-backend.c

@@ -14,8 +14,6 @@
 
 #include <sys/ioctl.h>
 
-extern const VhostOps user_ops;
-
 static int vhost_kernel_call(struct vhost_dev *dev, unsigned long int request,
                              void *arg)
 {

include/hw/virtio/vhost-backend.h

@@ -32,6 +32,8 @@ typedef struct VhostOps {
     vhost_backend_cleanup vhost_backend_cleanup;
 } VhostOps;
 
+extern const VhostOps user_ops;
+
 int vhost_set_backend_type(struct vhost_dev *dev,
                            VhostBackendType backend_type);