remove -Wno-header-guard workaround for old versions of NSS

I think it's safe to re-enable this warning now.  The bug was
fixed upstream in 3.15.1 and there have been several NSS upgrades since then.

BUG=255201

Review URL: https://codereview.chromium.org/1028863002

Cr-Commit-Position: refs/heads/master@{#321711}
1 file changed