Fix gcc 11 -Wzero-as-null-pointer-constant in wxMSW wxImageList

This warning was introduced by the recent 999340f288 (Check that
wxImageList::Create() is not called more than once, 2021-10-17), fix it
by using NULL for HIMAGELIST which is a pointer internally.

Closes #19298.

include/wx/msw/imaglist.h

@@ -32,7 +32,7 @@ public:
   // from icons), and the initial size of the list.
   wxImageList(int width, int height, bool mask = true, int initialCount = 1)
   {
-    m_hImageList = 0;
+    m_hImageList = NULL;
     Create(width, height, mask, initialCount);
   }
   virtual ~wxImageList();