[processui] Safety checks for platform != x11

Summary:
Starting Ksysguard on Wayland resulted in a crash as it unconditionally
calls into X11. This change performs some sanity checks and hides the
X11 specific columns from the view.

Test Plan: Run on Wayland, started and worked

Reviewers: mart, sebas

Subscribers: plasma-devel

Differential Revision: https://phabricator.kde.org/D859