outputview: use QProcess::nullDevice() as stdin for the build command
Summary:
Otherwise it will block forever when the build needs to run a command
that reads from stdin
Test Plan: Building still works for me
Reviewers: kfunk, mwolff
Reviewed By: mwolff
Subscribers: kdevelop-devel
Differential Revision: https://phabricator.kde.org/D1275