Avoid using std::log2 in workfactor.cpp
Avoid using std::log2 in workfactor.cpp

If a libc is missing any of the C99 math functions, old versions of
GCC disable *all* the std:: versions of the C99 functions, even those
that do exist.

GH #2235

randombit/botan,randombit/botan,randombit/botan,randombit/botan,randombit/botan