Having it in base.h pollutes the global namespace a bit and, in
particular, causes clang to give unhelpful suggestions in consuming
projects.

Change-Id: I6ca1a88bdd1701f0c49192a0df56ac0953c7067c
Reviewed-on: https://boringssl-review.googlesource.com/25464
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>