Make the wxDataViewItem(void*) constructor explicit.

Not having this as an implicit one made it possible to create
wxDataViewItem from any pointer without realizing it, leading to hard to
debug crashes later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68621 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/dataview.h

@@ -614,8 +614,9 @@ public:
     /**
         Constructor.
     */
-    wxDataViewItem(void* id = NULL);
+    wxDataViewItem();
     wxDataViewItem(const wxDataViewItem& item);
+    explicit wxDataViewItem(void* id);
     //@}
 
     /**