xen: Fix build error due to missing include

Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/xen_backend.h

@@ -5,6 +5,7 @@
 #include "sysemu.h"
 #include "net.h"
 #include "block_int.h"
+#include "blockdev.h"
 
 /* ------------------------------------------------------------- */