Update License Headers for Qt WebEngine to LGPLv3.

Change-Id: Idbe0eafb51d77cc00e3a93179b81770724d5bfaa
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Tuukka Turunen <tuukka.turunen@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>