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-maxp-table.hh

diff --git a/src/hb-ot-maxp-table.hh b/src/hb-ot-maxp-table.hh
index e270490..c2c90d1 100644
--- a/src/hb-ot-maxp-table.hh
+++ b/src/hb-ot-maxp-table.hh
@@ -52,7 +52,7 @@
   }
 
   /* We only implement version 0.5 as none of the extra fields in version 1.0 are useful. */
-  private:
+  protected:
   FixedVersion	version;		/* Version of the maxp table (0.5 or 1.0),
 					 * 0x00005000 or 0x00010000. */
   USHORT	numGlyphs;		/* The number of glyphs in the font. */