Minor doc issues.
[autofit] Improve tracing of style coverages.

* include/internal/fttrace.h: Add `afglobal' for tracing style
coverages.

* src/autofit/afglobal.c: Include FT_INTERNAL_DEBUG_H.
(FT_COMPONENT): Define.
(af_face_globals_compute_style_coverage): Trace `gstyles' array
data.
Minor.