exclude WebKit/JavaScriptCore from lupdate scan

there is no point in scanning it, and it's incredibly slow.

Change-Id: Ife2fe879b8285f18bb714804c532a68ed64f84d8
Reviewed-by: hjk <hjk121@nokiamail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>