Fix syntax error in interface headers

Although these headers are not supposed to be compiled, it's still
better to avoid at least the obvious typos in them.

Closes https://github.com/wxWidgets/wxWidgets/pull/2140

interface/wx/tarstrm.h

@@ -400,6 +400,6 @@ public:
     /**
         Assignment operator.
     */
-    wxTarEntry& operator operator=(const wxTarEntry& entry);
+    wxTarEntry& operator=(const wxTarEntry& entry);
 };