xfs_io: stat: treat statfs.f_flags as optional

xfs_io: stat: treat statfs.f_flags as optional

Kernels prior to 2.6.36 didn't contain statfs.f_flags.  Distros with
initial releases with kernels prior to this may not have updated
headers with this member.  Only attempt to print it if we have the
header with the member defined.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
[sandeen: define HAVE_STATFS_FLAGS in io/Makefile]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>