changed CClient type to align with znc-git/1.5

When compiling the module against the latest znc cloned from git (znc@78ea60e226e2b0f15b7e8d896ff18a22d0c59e2c) built with g++-4.8, results in these errors:

>$ znc-buildmod fish.cpp
Building "fish.so" for ZNC 1.5... fish.cpp: In member function âvirtual CModule::EModRet CFishMod::OnUserMsg(CString&, CString&)â:
fish.cpp:332:62: error: invalid initialization of reference of type âstd::vector<CClient*>&â from expression of type âconst std::vector<CClient*>â
    vector<CClient*>& vClients = this->m_pNetwork->GetClients();
                                                              ^
fish.cpp: In member function âvirtual CModule::EModRet CFishMod::OnUserAction(CString&, CString&)â:
fish.cpp:365:62: error: invalid initialization of reference of type âstd::vector<CClient*>&â from expression of type âconst std::vector<CClient*>â
    vector<CClient*>& vClients = this->m_pNetwork->GetClients();
                                                              ^
fish.cpp: In member function âvirtual CModule::EModRet CFishMod::OnUserNotice(CString&, CString&)â:
fish.cpp:398:62: error: invalid initialization of reference of type âstd::vector<CClient*>&â from expression of type âconst std::vector<CClient*>â
    vector<CClient*>& vClients = this->m_pNetwork->GetClients();
                                                              ^
[ !! ] Error while building "fish.so"

Simple fix, but it fixes all the above errors.