Change needed for ntl 6.2+, this is compatible with ntl 6.1

This was discussed on http://trac.sagemath.org/ticket/16882 and https://groups.google.com/forum/#!topic/flint-devel/YEsOO_lnQNs
The change is actually compatible with ntl 6.1 (flint 2.4.4 with that patch passes its test suite with ntl 6.1).