Re-added linux include path that was removed by mistake.

src/Parser/ParserOptions.cs

@@ -260,9 +260,12 @@ namespace CppSharp.Parser
                     AddSystemIncludeDirs($"{headersPath}/usr/include/c++/{version}");
                     AddSystemIncludeDirs($"{headersPath}/usr/include/c++/{version}/backward");
                     foreach (var tripple in tripples)
+                    {
+                        AddSystemIncludeDirs($"{headersPath}/usr/include/{tripple}/c++/{version}");
                         AddSystemIncludeDirs($"{headersPath}/usr/include/c++/{version}/{tripple}");
                     }
                 }
+            }
             foreach (var tripple in tripples)
             {
                 foreach (var version in versions)