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-icu.cc b/src/hb-icu.cc
index 8fc8c81..2f31a07 100644
--- a/src/hb-icu.cc
+++ b/src/hb-icu.cc
@@ -25,7 +25,7 @@
  * Red Hat Author(s): Behdad Esfahbod
  */
 
-#include "hb-private.h"
+#include "hb-private.hh"
 
 #include "hb-icu.h"