Adapt to freetype 2.6.  This patch copied from upstream, see:
https://bugzilla.mozilla.org/show_bug.cgi?id=1143411
https://hg.mozilla.org/mozilla-central/rev/afd840d66e6a

--- a/config/system-headers
+++ b/config/system-headers
@@ -415,6 +415,7 @@ freetype/ftbitmap.h
 freetype/ftxf86.h
 freetype.h
 ftcache.h
+ftfntfmt.h
 ftglyph.h
 ftsynth.h
 ftoutln.h
