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

Sorry for the noise, downstream custom builders.  Please adjust.
diff --git a/src/hb-null.hh b/src/hb-null.hh
index 91efee6..19886a5 100644
--- a/src/hb-null.hh
+++ b/src/hb-null.hh
@@ -27,7 +27,7 @@
 #ifndef HB_NULL_HH
 #define HB_NULL_HH
 
-#include "hb-private.hh"
+#include "hb.hh"
 
 
 /*