btrfs_tree.h: fix include from userland

This patch prepares the uapi export by fixing the following errors:

.../linux/btrfs_tree.h:283:2: error: #error "UUID items require BTRFS_UUID_SIZE == 16!"
 #error "UUID items require BTRFS_UUID_SIZE == 16!"

.../linux/btrfs_tree.h:390:12: error: ‘BTRFS_UUID_SIZE’ undeclared here (not in a function)
  __u8 uuid[BTRFS_UUID_SIZE];
            ^
.../linux/btrfs_tree.h:796:16: error: ‘BTRFS_DEV_STAT_VALUES_MAX’ undeclared here (not in a function)
  __le64 values[BTRFS_DEV_STAT_VALUES_MAX];

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>