staging/lustre/llite: Fix unused variable warning if !CONFIG_FS_POSIX_ACL

If CONFIG_FS_POSIX_ACL=n:

drivers/staging/lustre/lustre/llite/xattr.c: In function 'll_setxattr_common':
drivers/staging/lustre/lustre/llite/xattr.c:115:27: warning: unused variable 'rce' [-Wunused-variable]

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>