configure.ac: Fix uncorrect detection because of linux/btrfs.h

configure.ac: Fix uncorrect detection because of linux/btrfs.h

On old distros, we can't detect some btrfs structs because of
undeclared 'NULL' in btrfs_err_str function. This function has been
removed after kernel commit 68598d2ea8863 ("btrfs: remove
btrfs_err_str function from uapi/linux/btrfs.h").

Fix this bug in xfstests layer by adding stddef.h because NULL is
defined in stddef.h(it is in /usr/lib/gcc*/ directory).

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>