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

Sorry for the noise, downstream custom builders.  Please adjust.
diff --git a/src/hb-debug.hh b/src/hb-debug.hh
index 9056ffc..49663d2 100644
--- a/src/hb-debug.hh
+++ b/src/hb-debug.hh
@@ -27,8 +27,8 @@
 #ifndef HB_DEBUG_HH
 #define HB_DEBUG_HH
 
-#include "hb-private.hh"
-#include "hb-atomic-private.hh"
+#include "hb.hh"
+#include "hb-atomic.hh"
 #include "hb-dsalgs.hh"