Help: QLiteHtml: Ensure C++14 is used

Old compilers (e.g. gcc5.3) may not use C++14 as
default.

Change-Id: Ifc7fb9afe23b4aaab0d2f0ee42baca28d3087cfc
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/help/qlitehtml/qlitehtml.qbs

@@ -26,6 +26,7 @@ Product {
     }
     cpp.optimization: "fast"
     cpp.warningLevel: "none"
+    cpp.cxxLanguageVersion: "c++14"
 
     Export {
         Depends { name: "cpp" }