Require libinput and udev

Summary:
The main reason for not having it as a mandatory dependency was that BSD
doesn't support it. But as I learned recently it is available on our CI
system. So BSDs have support now.

Even more it showed that the code doesn't compile if the dependency is
missing. And there's one thing I hate: broken build configuration
options.

So let's make UDEV and libinput a required dependency and get rid of the
problems.

Test Plan: Compiles

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10057