Summary:
Otherwise we can find X11 without the required xf86vmode and fail at
link time instead of configure time.

Current CI output:

[2019-12-25T21:45:30.854Z] CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
[2019-12-25T21:45:30.854Z] Please set them or make sure they are set and tested correctly in the CMake files:
[2019-12-25T21:45:30.854Z] X11_Xxf86vm_LIB (ADVANCED)
[2019-12-25T21:45:30.854Z] linked by target "kcm_kgamma" in directory /home/jenkins/workspace/Plasma/kgamma5/stable-kf5-qt5 SUSEQt5.12/kcmkgamma
[2019-12-25T21:45:30.854Z]
[2019-12-25T21:45:30.854Z] -- Configuring incomplete, errors occurred!

Test Plan:
Configured
Added a deliberate typo to my required X11 lines
Configure failed

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26223