Fix race condition with libinput events on startup

Summary:
In some cases, it's possible that libinput wrote device added events to
the file descriptor before the connection to handle those events was
in-place. This resulted in a compositor without any input devices.

Test Plan:
Ran a wayland session. In about 60% of all cases, no input was
possible. kwin_libinput showed the enumeration of all devices correctly,
but KWin::LibInput::Context did not have any m_devices.
After this change, this did not appear anymore.

Reviewers: #plasma, graesslin, davidedmundson

Reviewed By: #plasma, graesslin, davidedmundson

Subscribers: anthonyfieroni, ngraham, kwin, #kwin, plasma-devel

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8888