Make GTK callbacks passed to GTKConnectWidget() extern "C".

This fixes (harmless but annoying) warnings about mixing C++ and C linkage
function pointers from Sun compiler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69565 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/window.cpp

@@ -2444,7 +2444,8 @@ void wxWindowGTK::PostCreation()
         gtk_widget_show( m_widget );
 }
 
-unsigned long wxWindowGTK::GTKConnectWidget(const char *signal, void (*callback)())
+unsigned long
+wxWindowGTK::GTKConnectWidget(const char *signal, wxGTKCallback callback)
 {
     return g_signal_connect(m_widget, signal, callback, this);
 }