Fix build with freetype 2.5.1.
https://bugzilla.xamarin.com/show_bug.cgi?id=16521
--- src/gdiplus-private.h.orig	2011-12-02 11:23:12.000000000 -0600
+++ src/gdiplus-private.h	2013-11-29 17:26:34.000000000 -0600
@@ -30,7 +30,8 @@
 #include <stdio.h>
 #include <math.h>
 #include <glib.h>
-#include <freetype/tttables.h>
+#include <ft2build.h>
+#include FT_TRUETYPE_TABLES_H
 #include <pthread.h>
 #include <unistd.h>