unicode: fix .gitignore for generated utfdata file

Commit 2b3d0478 ("unicode: Add utf8-data module") changed the
generated utf8data file from 'utf8data.h' to 'utf8data.c', but didn't
change the comments or the .gitignore to match.

The comments should be updated too, but at least they don't cause any
visible breakage.  But the gitignore file needs changing to avoid git
complaining about untracked files.

Fixes: 2b3d0478

 ("unicode: Add utf8-data module")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>