Make it an error to include non-top-level headers

Users should #include <hb.h> (or hb-ft.h, hb-glib.h, etc), but
never things like hb-shape.h directly.  This makes it easier to
refactor headers later on without breaking compatibility.

src/hb-unicode.h

diff --git a/src/hb-unicode.h b/src/hb-unicode.h
index 13886df..205e4c7 100644
--- a/src/hb-unicode.h
+++ b/src/hb-unicode.h
@@ -28,6 +28,10 @@
  * Google Author(s): Behdad Esfahbod
  */
 
+#ifndef HB_H_IN
+#error "Include <hb.h> instead."
+#endif
+
 #ifndef HB_UNICODE_H
 #define HB_UNICODE_H