Remove unneeded include.

Remove unneeded include.

This include was added during the MPX/XI2 development cycle, likely when the
headers weren't in an acceptable state and clients needed stuff defined in
the proto headers. Normal clients shouldn't need the proto headers though
and xinput builds fine without it.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>