check-style should not complain about code in icu directories

check-style should not complain about code in icu directories
https://bugs.webkit.org/show_bug.cgi?id=53929

Reviewed by Darin Adler.

Skip style checking of imported ICU headers to avoid a deluge of style issues
as they do not conform to the WebKit Code Style guideline. We want to preserve
the formatting of these headers so as to make it straightforward to merge
changes to them, if necessary.

* Scripts/webkitpy/style/checker.py:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@209902 268f45cc-cd09-0410-ab3c-d52691b4dbfc