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; }
2 files changed