gtk+: fix test

Formula/gtk+.rb

@@ -78,6 +78,7 @@ class Gtkx < Formula
     gdk_pixbuf = Formula["gdk-pixbuf"]
     gettext = Formula["gettext"]
     glib = Formula["glib"]
+    harfbuzz = Formula["harfbuzz"]
     libpng = Formula["libpng"]
     pango = Formula["pango"]
     pixman = Formula["pixman"]
@@ -90,6 +91,7 @@ class Gtkx < Formula
       -I#{gettext.opt_include}
       -I#{glib.opt_include}/glib-2.0
       -I#{glib.opt_lib}/glib-2.0/include
+      -I#{harfbuzz.opt_include}/harfbuzz
       -I#{include}/gtk-2.0
       -I#{libpng.opt_include}/libpng16
       -I#{lib}/gtk-2.0/include