vfs_subr.c: fix signature mismatch for close method

Reviewed by:    jhb
Differential Revision:  https://reviews.freebsd.org/D49166

sys/kern/vfs_subr.c

@@ -5374,7 +5374,7 @@ vn_pollrecord(struct vnode *vp, struct thread *td, int events)
 /*
  * Routine to create and manage a filesystem syncer vnode.
  */
-#define sync_close ((int (*)(struct  vop_close_args *))nullop)
+static int	sync_close(struct vop_close_args *);
 static int	sync_fsync(struct  vop_fsync_args *);
 static int	sync_inactive(struct  vop_inactive_args *);
 static int	sync_reclaim(struct  vop_reclaim_args *);
@@ -5468,6 +5468,12 @@ vfs_deallocate_syncvnode(struct mount *mp)
 		vrele(vp);
 }
 
+static int
+sync_close(struct vop_close_args *ap __unused)
+{
+	return (0);
+}
+
 /*
  * Do a lazy sync of the filesystem.
  */