message: core: make sure line number is in decimal
  Without this, some higher line numbers were suddenly displayed in hex.
diff: @@ -106,7 +106,7 @@ public:
  set_color(Color::RESET);
  std::cout << _s.str();
  - std::cout << " (" << _caller_filename << ":" << _caller_filenumber << ")";
  + std::cout << " (" << _caller_filename << ":" << std::dec << _caller_filenumber << ")";
  std::cout << std::endl;
  #endif
  