Rename hb-*private.hh to hb-*.hh

Sorry for the noise, downstream custom builders.  Please adjust.
diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index 1b916a5..a7ba85f 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -28,8 +28,8 @@
 
 #define HB_SHAPER coretext
 
-#include "hb-private.hh"
-#include "hb-shaper-impl-private.hh"
+#include "hb.hh"
+#include "hb-shaper-impl.hh"
 
 #include "hb-coretext.h"
 #include <math.h>