[Touchpad KCM] Fix libinput-less build and warnings

Summary:
If the X.Org libinput dev packages are not found, do not
build the X11 libinput backend. This way the build succeeds.

Fix also some minor warnings.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21220