lint: support cpplint 2.0

[wx257osn2]

Current poac code fails poac lint with cpplint 2.0.0. This PR fixes this:

$ cpplint --version
Cpplint fork (https://github.com/cpplint/cpplint)
cpplint 2.0.0
Python 3.12.3 (main, Nov  6 2024, 18:32:19) [GCC 13.2.0]
$ poac-out/debug/poac lint --exclude srcOld --exclude testsOld
   Linting poac
src/BuildConfig.cc:19:  <filesystem> is an unapproved C++17 header.  [build/c++17] [5]
Done processing src/BuildConfig.cc
src/Command.cc:235:  Add #include <string> for string  [build/include_what_you_use] [4]
Done processing src/Command.cc
src/Command.hpp:4:  <filesystem> is an unapproved C++17 header.  [build/c++17] [5]
Done processing src/Command.hpp
src/Rustify/Aliases.hpp:4:  <filesystem> is an unapproved C++17 header.  [build/c++17] [5]
Done processing src/Rustify/Aliases.hpp
Total errors found: 4
Error: 'poac lint' failed with exit code `1`

[ken-matsui]

Thanks!