libattr is no longer needed

wimlib only uses the extended attributes interface on Linux, where it
appears it is now safe to assume the functions are present in libc (see:
http://lists.nongnu.org/archive/html/acl-devel/2012-04/msg00001.html).

Note: the setfattr program from the "attr" package is still required to
run the NTFS-3G test script.