Make data members of various OpenType structs protected instead of private

Should fix warnings generated when building with -Wunused-private-field.
Based on patch from Jonathan Kew.

src/hb-ot-hhea-table.hh

diff --git a/src/hb-ot-hhea-table.hh b/src/hb-ot-hhea-table.hh
index 2eea05a..24f8bdc 100644
--- a/src/hb-ot-hhea-table.hh
+++ b/src/hb-ot-hhea-table.hh
@@ -47,7 +47,7 @@
     return TRACE_RETURN (c->check_struct (this) && likely (version.major == 1));
   }
 
-  private:
+  protected:
   FixedVersion	version;		/* 0x00010000 for version 1.0. */
   FWORD		ascender;		/* Typographic ascent. <a
 					 * href="http://developer.apple.com/fonts/TTRefMan/RM06/Chap6hhea.html">