LICENSE: add missing file

Every repo needs to explicitly declare this.

BUG=None
TEST=CQ passes

Change-Id: Id62bd374c8989ecd15796670600d1a2d9d4fbce2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/libevdev/+/2170908
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>