Use ptrdiff_t as wxVector::difference_type instead of size_t

difference_type must be signed, using size_t was clearly a mistake.

include/wx/vector.h

@@ -160,7 +160,7 @@ class wxVector
 
 public:
     typedef size_t size_type;
-    typedef size_t difference_type;
+    typedef ptrdiff_t difference_type;
     typedef T value_type;
     typedef value_type* pointer;
     typedef const value_type* const_pointer;