staging/lustre: fix build error when CONFIG_FS_POSIX_ACL is off

We need to include <linux/posix_acl_xattr.h> regardless of
CONFIG_FS_POSIX_ACL is set or not. Otherwise build fails as
reported by kbuild robot:

>> drivers/staging/lustre/lustre/llite/file.c:2965:2: error: implicit declaration of function 'posix_acl_dup' [-Werror=implicit-function-declaration]
     acl = posix_acl_dup(lli->lli_posix_acl);
<many similar errors omitted>

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Peng Tao <bergwolf@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>