Clean up config-X11.h

Summary:
Khotkeys only requires libX11 and the optional Xtst component.

Report whether the optional Xtst component was found.

Before: -- Found X11: /usr/include
After: -- Found X11: /usr/include found components: Xtst

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25275