Add missing wxUSE_XXX checks to persistent adapters headers

Make sure these headers can be compiled even when the control they're
written for is not available in the build. Including them in this case
doesn't make much sense, of course, but not giving any errors is still
nicer and consistent with the rest of wx headers.

include/wx/persist/combobox.h

@@ -12,6 +12,8 @@
 
 #include "wx/persist/window.h"
 
+#if wxUSE_COMBOBOX
+
 #include "wx/combobox.h"
 
 #define wxPERSIST_COMBOBOX_KIND wxASCII_STR("Combobox")
@@ -96,4 +98,6 @@ inline wxPersistentObject *wxCreatePersistentObject(wxComboBox* combobox)
     return new wxPersistentComboBox(combobox);
 }
 
+#endif // wxUSE_COMBOBOX
+
 #endif // _WX_PERSIST_COMBOBOX_H_