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-ot-layout-gdef-private.hh b/src/hb-ot-layout-gdef-private.hh
index 4172a7c..242b33a 100644
--- a/src/hb-ot-layout-gdef-private.hh
+++ b/src/hb-ot-layout-gdef-private.hh
@@ -31,7 +31,7 @@
 
 #include "hb-ot-layout-common-private.hh"
 
-#include "hb-font-private.h"
+#include "hb-font-private.hh"
 
 HB_BEGIN_DECLS