Compilation fix for wxUSE_STL build: another missing _str().

Use utf8_str() to convert wxString to GTK+ string instead of relying on
implicit conversion which doesn't exist when wxUSE_STL==1 (and also when using
wchar_t-based Unicode build).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62686 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/infobar.cpp

@@ -209,9 +209,9 @@ GtkWidget *wxInfoBar::GTKAddButton(wxWindowID btnid, const wxString& label)
     GtkWidget *button = gtk_info_bar_add_button
                         (
                             GTK_INFO_BAR(m_widget),
-                            label.empty()
+                            (label.empty()
                                 ? GTKConvertMnemonics(wxGetStockGtkID(btnid))
-                                : label,
+                                : label).utf8_str(),
                             btnid
                         );