gtksourceview4: fix test

Formula/gtksourceview4.rb

@@ -44,6 +44,7 @@ class Gtksourceview4 < Formula
     gettext = Formula["gettext"]
     glib = Formula["glib"]
     gtkx3 = Formula["gtk+3"]
+    harfbuzz = Formula["harfbuzz"]
     libepoxy = Formula["libepoxy"]
     libpng = Formula["libpng"]
     pango = Formula["pango"]
@@ -59,6 +60,7 @@ class Gtksourceview4 < Formula
       -I#{glib.opt_include}/glib-2.0
       -I#{glib.opt_lib}/glib-2.0/include
       -I#{gtkx3.opt_include}/gtk-3.0
+      -I#{harfbuzz.opt_include}/harfbuzz
       -I#{include}/gtksourceview-4
       -I#{libepoxy.opt_include}
       -I#{libpng.opt_include}/libpng16