simple example not compiling

[correaa]

This is the first example in the README (called http.cpp here).

$ g++ -std=c++17 http.cpp -Wfatal-errors                                                                                                                 
In file included from http.cpp:1:
belle/include/belle.hh:618:24: error: ‘OB::Belle::websocket::role_type’ has not been declared
  618 |   friend void teardown(websocket::role_type,
      |                        ^~~~~~~~~

$ g++ -std=c++17 http.cpp -Wfatal-errors                                                                                                                    In file included from http.cpp:1:
belle/include/belle.hh:618:24: error: ‘OB::Belle::websocket::role_type’ has not been declared
  618 |   friend void teardown(websocket::role_type,
      |                        ^~~~~~~~~

Using Boost 1.71