linux-user: Protect btrfs ioctl target definitions

linux-user: Protect btrfs ioctl target definitions

Target definitions of btrfs ioctls in 'syscall_defs.h' use
the value BTRFS_IOCTL_MAGIC that is defined header 'btrfs.h'.
This header is not available in kernel versions before 3.9.
For that reason, these target ioctl definitions should be
enwrapped in an #ifdef directive to check whether the 'btrfs.h'
header is available as to not cause build errors on older
Linux systems.

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200905163802.2666-1-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>