Declare a local variable in filefn.cpp as const

No real changes, just make a variable which is set only once const.

src/common/filefn.cpp

@@ -1607,7 +1607,7 @@ static bool wxCheckWin32Permission(const wxString& path, DWORD access)
         return false;
     }
 
-    HANDLE h = ::CreateFile
+    const HANDLE h = ::CreateFile
                  (
                     path.t_str(),
                     access,