Include calloc() etc. Wasn't compiling with recent GCC versions.

Without `cstdlib`, compilation of e.g. NERsuite against `crfsuite.hpp` was failing with

    /usr/local/include/crfsuite.hpp:332: error: ‘calloc’ was not declared in this scope
    /usr/local/include/crfsuite.hpp:353: error: ‘exit’ was not declared in this scope
    /usr/local/include/crfsuite.hpp:361: error: ‘atof’ was not declared in this scope
    /usr/local/include/crfsuite.hpp:374: error: ‘free’ was not declared in this scope

this line fixes the issue.