Fixed build error with ZNC 1.6.2

[grefab]

When building the module with znc-buildmod with ZNC 1.6.2 this produced an error:

Building "colloquy.so" for ZNC 1.6.2... colloquy.cpp: In member function ‘bool CColloquyMod::Push(const CString&, const CString&, const CString&, bool, int)’: colloquy.cpp:778:58: error: invalid initialization of reference of type ‘std::vector<CClient*>&’ from expression of type ‘const std::vector<CClient*>’ vector<CClient*>& vpClients = GetNetwork()->GetClients();

Seems like GetClients() returns a const reference and the module was not handling that properly. Since vpClients is used read-only making it const did not break anything and it works that way.