Bracket MacNewFile() with __WXMAC__ checks in the docview sample.

This fixes compilation in C++11 mode: "override" results in an error on
non-Mac platforms because this method doesn't exist there and so can't be
overridden.

Closes #16903.

samples/docview/docview.cpp

@@ -152,10 +152,12 @@ bool MyApp::OnCmdLineParsed(wxCmdLineParser& parser)
     return wxApp::OnCmdLineParsed(parser);
 }
 
+#ifdef __WXMAC__
 void MyApp::MacNewFile()
 {
     wxDocManager::GetDocumentManager()->CreateNewDocument();
 }
+#endif // __WXMAC__
 
 bool MyApp::OnInit()
 {