Include standard headers to use bool and size_t

[dmitry-kabanov]

Hi, I'd like to propose a small fix that fixing the compilation error about unknown type bool and removes linting warnings about size_t when hashmap_base.h is opened.

The first is an actual error, at least with Clang 14 on macOS. The warning about size_t does not lead to errors, as stddefs.h in hashmap.h is included, however, inside Neovim, linting with clang++ (and C++14 standard) leads to these warnings.

[dleeds-cpi]

Thanks for the fix!