Added missing PDF global setting

The [doc](http://wkhtmltopdf.org/libwkhtmltox/pagesettings.html#pagePdfGlobal) states that there is a `outlineDepth` property accessible on the global setting object but setting or getting the setting will not work as the `WKHTMLTOPDF_REFLECT` for that property is missing.

src/lib/pdfsettings.cc

@@ -118,6 +118,7 @@ struct DLL_LOCAL ReflectImpl<PdfGlobal>: public ReflectClass {
 		WKHTMLTOPDF_REFLECT(copies);
 		WKHTMLTOPDF_REFLECT(collate);
 		WKHTMLTOPDF_REFLECT(outline);
+		WKHTMLTOPDF_REFLECT(outlineDepth);
 		WKHTMLTOPDF_REFLECT(dumpOutline);
 		WKHTMLTOPDF_REFLECT(out);
 		WKHTMLTOPDF_REFLECT(documentTitle);