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