Fix recursion in calling const getters

We have to explicitly call non-const getter to avoid recursive calls of the const getter.

include/wx/propgrid/propgridiface.h

@@ -305,7 +305,7 @@ public:
 
     const wxPGProperty* GetFirst( int flags = wxPG_ITERATE_ALL ) const
     {
-        return GetFirst(flags);
+        return const_cast<wxPropertyGridInterface*>(this)->GetFirst(flags);
     }
 
     // Returns pointer to a property with given name (case-sensitive).

include/wx/propgrid/propgridpagestate.h

@@ -412,7 +412,7 @@ public:
 
     const wxPGProperty* GetLastItem( int flags = wxPG_ITERATE_DEFAULT ) const
     {
-        return GetLastItem(flags);
+        return const_cast<wxPropertyGridPageState*>(this)->GetLastItem(flags);
     }
 
     // Returns currently selected property.