[Qt] Build fix. Enabling c++11 for Qt WK2 after r149259.
https://bugs.webkit.org/show_bug.cgi?id=115345

Reviewed by Anders Carlsson.

* WebKit2.pri:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149291 268f45cc-cd09-0410-ab3c-d52691b4dbfc