Add wxGridBlocks::iterator::operator->()

An iterator should have this operator defined too, and not just
operator*() as it used to.

interface/wx/grid.h

@@ -2045,6 +2045,7 @@ public:
         iterator();
 
         const wxGridBlockCoords& operator*() const;
+        const wxGridBlockCoords* operator->() const;
 
         iterator& operator++();
         iterator operator++(int);