fix: reset doctor output color before exiting

Summary:
Without this patch the blue from the "primary" output 'leaks' into the
prompt of some shells (e.g. busybox ash).

Test Plan: Run "kscreen-doctor -o" in ash and observe that the prompt isn't blue anymore afterwards.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: romangg, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27383