Bump xcconfigs to remove USER_HEADER_SEARCH_PATHS

See https://github.com/jspahrsummers/xcconfigs/pull/31.
https://github.com/jspahrsummers/xcconfigs/compare/6c7a02a43954c74d2be1d2fcf7bfd65a263e7c52...6c64238566ef1126f1b7994aa9e6719229c50451