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