std::abs used without including header in extended_p_square.hpp

`std::abs` needs either `` or `` but neither is included. This results in a particularly hairy ambiguous overload problem on some old versions of gcc.

It also may be better to use `std::fabs`, which is intended for [this use case](https://github.com/boostorg/accumulators/blob/14c13370602fe86d134a948943958cab0921ce9c/include/boost/accumulators/statistics/extended_p_square.hpp#L200)