Add missing copyright header to speculation_rule_set_test.cc.

Bug: 1173646
Change-Id: I108e618409c21668ed5d0e26105b498a85cddcbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2774165


Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Reviewed-by: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/master@{#864685}