Set default scope for ProblemStore

Summary:
This commit fixes uninitialized scope value for ProblemStore class. Old version leads to ASSERT during tests run:

QFATAL : TestFilteredProblemStore::testNoGrouping() ASSERT: "d->m_documents" in file /home/jenkins/sources/kdevplatform/kf5-qt5/shell/problemstore.cpp, line 228
FAIL! : TestFilteredProblemStore::testNoGrouping() Received a fatal error.

Test Plan: Tested on master branch.

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4035