include <cmath> in flatten_internal.hh
include <cmath> in flatten_internal.hh

Fixes a failure to build with latest GCC.

nathanielbaxter/libminizinc,nathanielbaxter/libminizinc,nathanielbaxter/libminizinc,jjdekker/libminizinc,jjdekker/libminizinc,jjdekker/libminizinc,nathanielbaxter/libminizinc,jjdekker/libminizinc