Add a missing #include <string> in lefout.h

[ax3ghazy]

This is needed for https://github.com/The-OpenROAD-Project/OpenDB/blob/master/include/opendb/lefout.h#L84.

[ax3ghazy]

@maliberty: Is this getting merged? Without it, building fails on my system:

OpenROAD/src/OpenDB/src/lefout/lefout.cpp: In member function ‘void odb::lefout::writePropertyDefinitions(odb::dbLib*)’:
OpenROAD/src/OpenDB/src/lefout/lefout.cpp:930:43: error: cannot convert ‘std::unordered_map<std::__cxx11::basic_string<char>, short int>’ to ‘int ’
  930 |     writeObjectPropertyDefinitions(layer, propertiesMap);
      |                                           ^~~~~~~~~~~~~
      |                                           |
      |                                           std::unordered_map<std::__cxx11::basic_string<char>, short int>