Fix build when ICU is disabled

Thanks FFForever for report

src/znc.cpp

@@ -2074,11 +2074,13 @@ void CZNC::ResumeConnectQueue() {
 
 void CZNC::ForceEncoding() {
     m_uiForceEncoding++;
+#ifdef HAVE_ICU
     for (Csock* pSock : GetManager()) {
         if (pSock->GetEncoding().empty()) {
             pSock->SetEncoding("UTF-8");
         }
     }
+#endif
 }
 void CZNC::UnforceEncoding() { m_uiForceEncoding--; }
 bool CZNC::IsForcingEncoding() const { return m_uiForceEncoding; }