Fix bug 375007 - Asserts when pressing "Show Imports" button in problems tool view
Summary: Fix asserts by adding missing DUChainReadLocker to KDevelop::WatchedDocumentSetPrivate::getImportsFromDUChain.
Test Plan: tested on master branch
Reviewers: KDevelop, kfunk
Reviewed By: KDevelop, kfunk
Subscribers: kdevelop-devel
Differential Revision: https://phabricator.kde.org/D4152