lang/maude: Fix build with Clang 6

../../src/Core/termSet.hh:35:15: error: ISO C++11 does not allow access declarations; use using declarations instead

PointerSet::cardinality;
            ^

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/maude-2.6_1.log