gtkextra: fix test

Formula/gtkextra.rb

@@ -41,6 +41,7 @@ class Gtkextra < Formula
     gettext = Formula["gettext"]
     glib = Formula["glib"]
     gtkx = Formula["gtk+"]
+    harfbuzz = Formula["harfbuzz"]
     libpng = Formula["libpng"]
     pango = Formula["pango"]
     pixman = Formula["pixman"]
@@ -55,6 +56,7 @@ class Gtkextra < Formula
       -I#{glib.opt_lib}/glib-2.0/include
       -I#{gtkx.opt_include}/gtk-2.0
       -I#{gtkx.opt_lib}/gtk-2.0/include
+      -I#{harfbuzz.opt_include}/harfbuzz
       -I#{include}/gtkextra-3.0
       -I#{libpng.opt_include}/libpng16
       -I#{pango.opt_include}/pango-1.0