fix ‘runtime_error’ is not a member of ‘std’ build error

[cirosantilli]

On Ubuntu 23.10 build was failing with:

support/lockedpool.cpp: In member function ‘void Arena::free(void*)’:
support/lockedpool.cpp:99:20: error: ‘runtime_error’ is not a member of ‘std’
   99 |         throw std::runtime_error("Arena: invalid or double free");

[kallewoof]

Thanks!