Implement wxWebView::GetVersionInfo() for macOS

Return the operating system version as the WebKit
version used by WKWebView is bound to the OS.
(Also works for iOS)

include/wx/osx/webview_webkit.h

@@ -122,6 +122,7 @@ public:
                               long style = 0,
                               const wxString& name = wxASCII_STR(wxWebViewNameStr))
     { return new wxWebViewWebKit(parent, id, url, pos, size, style, name); }
+    virtual wxVersionInfo GetVersionInfo() wxOVERRIDE;
 };
 
 #endif // wxUSE_WEBVIEW && wxUSE_WEBVIEW_WEBKIT