Make linux/input.h compile time optional

Summary:
Instead of hard depending on the include of linux/input.h we check
whether that include file exists and properly ifdef all usages.

Unfortunately there is no replacement for those parts which do mapping
of input event codes.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2344