Fix build with -Dexperimental_api=true

../src/hb-style.cc:102:32: error: no matching function for call to 'hb_style_get_value'
    return _hb_angle_to_ratio (hb_style_get_value (HB_STYLE_TAG_SLANT_ANGLE));
                               ^~~~~~~~~~~~~~~~~~
../src/hb-style.cc:99:1: note: candidate function not viable: requires 2 arguments, but 1 was provided
hb_style_get_value (hb_font_t *font, hb_tag_t tag)
^

and

../src/hb-style.cc:80:1: error: unused function '_hb_ratio_to_angle' [-Werror,-Wunused-function]
_hb_ratio_to_angle (float r)
^
1 file changed