include: Fix an idl string type.

include/axextend.idl

@@ -266,7 +266,7 @@ interface IFilterMapper2 : IUnknown
 
     HRESULT UnregisterFilter
         ( [in] const CLSID *pclsidCategory,
-          [in] const OLECHAR *szInstance,
+          [in] LPCOLESTR szInstance,
           [in] REFCLSID Filter
         );
 
@@ -275,7 +275,7 @@ interface IFilterMapper2 : IUnknown
           [in] LPCWSTR Name,
           [in, out] IMoniker **ppMoniker,
           [in] const CLSID *pclsidCategory,
-          [in] const OLECHAR *szInstance,
+          [in] LPCOLESTR szInstance,
           [in] const REGFILTER2 *prf2
         );