btrfs: send: drop __KERNEL__ ifdef from send.h

We don't need this ifdef as the header file is not shared, the protocol
definition used by userspace should be from libbtrfs or libbtrfsutil.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/send.h

@@ -155,8 +155,6 @@ enum {
 	BTRFS_SEND_A_MAX		= 31,
 };
 
-#ifdef __KERNEL__
 long btrfs_ioctl_send(struct inode *inode, struct btrfs_ioctl_send_args *arg);
-#endif
 
 #endif