Fix the build when implicit wxString conversions are disabled

This corrects a bug introduced in 5d256988be (Add wxMSWFormatMessage()
and use it from other places, 2021-01-09).

src/common/log.cpp

@@ -1143,7 +1143,8 @@ wxString wxSysErrorMsgStr(unsigned long nErrCode)
 const wxChar *wxSysErrorMsg(unsigned long nErrCode)
 {
     static wxChar s_szBuf[1024];
-    wxStrlcpy(s_szBuf, wxSysErrorMsgStr(nErrCode), WXSIZEOF(s_szBuf));
+    wxStrlcpy(s_szBuf, (const wxChar*)wxSysErrorMsgStr(nErrCode).c_str(),
+              WXSIZEOF(s_szBuf));
     return s_szBuf;
 }