Change to the (always intended) MIT license

Change to the (always intended) MIT license

Due to what must've been a copy/paste error many years ago, the license text
for libevdev wasn't actually the MIT license. Let's rectify this, it was
always MIT intended anyway.

To make this more obvious and reduce the chance of copy/paste mistakes, use
the SPDX license identifier in the various source files. The two installed
public header files have the full license text.

All contributors with copyrightable contributions have ACKed the license
change to MIT, either in the MR directly [1] or privately in reply to an
email.

[1] https://gitlab.freedesktop.org/libevdev/libevdev/-/merge_requests/69

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Alexander Dahl <ada@thorsis.com>
Acked-by: Andreas Pokorny <andreas.pokorny@canonical.com>
Acked-by: Armin K <krejzi@email.com>
Acked-by: Benjamin Tissoires <btissoir@redhat.com>
Acked-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: Emmanuele Bassi <ebassi@gnome.org>
Acked-by: Gaetan Nadon <memsize@videotron.ca>
Acked-by: George Thomas <georgefsthomas@gmail.com>
Acked-by: Michael Forney <mforney@mforney.org>
Acked-by: Nayan Deshmukh <nayan26deshmukh@gmail.com>
Acked-by: Niclas Zeising <zeising@daemonic.se>
Acked-by: Owen W. Taylor <otaylor@fishsoup.net>
Acked-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Ran Benita <ran234@gmail.com>
Acked-by: Rosen Penev <rosenp@gmail.com>
Acked-by: Scott Jann <sjann@knight-rider.org>
Acked-by: Thilo Schulz <thilo@tjps.eu>
Acked-by: polyphemus <rolfmorel@gmail.com>