Fix sign compare warnings

../../third_party/harfbuzz-ng/src/src/hb-map.hh(56,45):  warning: comparison of integers of different signs: 'const hb_codepoint_t' (aka 'const unsigned int') and 'hb_map_t::(anonymous enum at ../../third_party/harfbuzz-ng/src/src/hb-map.hh:169:3)' [-Wsign-compare]
    bool is_unused () const    { return key == INVALID; }

src/hb-map.hh

@@ -166,7 +166,7 @@ struct hb_map_t
   hb_codepoint_t operator [] (unsigned int key) const
   { return get (key); }
 
-  enum { INVALID = HB_MAP_VALUE_INVALID };
+  static const hb_codepoint_t INVALID = HB_MAP_VALUE_INVALID;
 
   void clear ()
   {

src/hb-set.hh

@@ -666,7 +666,7 @@ struct hb_set_t
     return INVALID;
   }
 
-  static  const hb_codepoint_t INVALID = HB_SET_VALUE_INVALID;
+  static const hb_codepoint_t INVALID = HB_SET_VALUE_INVALID;
 
   /*
    * Iterator implementation.