Override IsSeekable() in wxString{Input,Output}Stream to return true.

These streams are seekable but somehow IsSeekable() was never overridden for
them resulting in test failures in Input_SeekI().

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63248 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/sstream.h

@@ -28,6 +28,7 @@ public:
     wxStringInputStream(const wxString& s);
 
     virtual wxFileOffset GetLength() const;
+    virtual bool IsSeekable() const { return true; }
 
 protected:
     virtual wxFileOffset OnSysSeek(wxFileOffset ofs, wxSeekMode mode);
@@ -76,6 +77,8 @@ public:
     // get the string containing current output
     const wxString& GetString() const { return *m_str; }
 
+    virtual bool IsSeekable() const { return true; }
+
 protected:
     virtual wxFileOffset OnSysTell() const;
     virtual size_t OnSysWrite(const void *buffer, size_t size);