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

Sorry for the noise, downstream custom builders.  Please adjust.
diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 25c3e05..a335df3 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -30,8 +30,8 @@
 #define _POSIX_C_SOURCE 200809L
 #endif
 
-#include "hb-private.hh"
-#include "hb-blob-private.hh"
+#include "hb.hh"
+#include "hb-blob.hh"
 
 #ifdef HAVE_SYS_MMAN_H
 #ifdef HAVE_UNISTD_H