Separate face source code from font

Makes documentation / introspection easier.

src/hb.h

diff --git a/src/hb.h b/src/hb.h
index 52c479c..5a6e9fd 100644
--- a/src/hb.h
+++ b/src/hb.h
@@ -31,6 +31,7 @@
 #include "hb-blob.h"
 #include "hb-buffer.h"
 #include "hb-common.h"
+#include "hb-face.h"
 #include "hb-font.h"
 #include "hb-set.h"
 #include "hb-shape.h"