Add include to fix std::counting_semaphore usage

[cho-m]

Issue

Build failure seen on macOS 26 when building with Clang:

In file included from problems/vrp.cpp:10:
./problems/vrp.h:207:10: error: no member named 'counting_semaphore' in namespace 'std'
  207 |     std::counting_semaphore<32> semaphore(actual_nb_threads);
      |     ~~~~~^
./problems/vrp.h:207:33: error: use of undeclared identifier 'semaphore'
  207 |     std::counting_semaphore<32> semaphore(actual_nb_threads);
      |                                 ^

Seemed like simple change so directly opened pull request, but can create & reference a separate issue if preferred.

Tasks

• review