xen: fix poll misannotation

Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

drivers/xen/pvcalls-front.h

@@ -20,7 +20,7 @@ int pvcalls_front_recvmsg(struct socket *sock,
 			  struct msghdr *msg,
 			  size_t len,
 			  int flags);
-unsigned int pvcalls_front_poll(struct file *file,
+__poll_t pvcalls_front_poll(struct file *file,
 				struct socket *sock,
 				poll_table *wait);
 int pvcalls_front_release(struct socket *sock);