libinput: Use #pragma once instead of explicit define guards

The #pragma once directive is commonly used by modern cpp code, and
support for its use in Andorid is proposed in go/droidcppstyle.

Bug: 245989146
Test: Build, Presubmit
Change-Id: Ib7852f1a343864dff46e7bc616d707bc7d5bf512
13 files changed