Rename all private sources and headers to C++ files

So we can liberally use the simple features of C++ that parts of the
codebase is already using.
diff --git a/src/hb-blob.cc b/src/hb-blob.cc
index 511455b..e2af8b7 100644
--- a/src/hb-blob.cc
+++ b/src/hb-blob.cc
@@ -24,9 +24,9 @@
  * Red Hat Author(s): Behdad Esfahbod
  */
 
-#include "hb-private.h"
+#include "hb-private.hh"
 
-#include "hb-blob-private.h"
+#include "hb-blob-private.hh"
 
 #ifdef HAVE_SYS_MMAN_H
 #ifdef HAVE_UNISTD_H