Reportedly some combination of C++ modules and old clang gets upset.
That seems an inadvisable combination, but including headers under
extern "C" is rude, so fix it.
Change-Id: I12f873e1be41697b67f2b1145387a3c6fc769c28
Reviewed-on: https://boringssl-review.googlesource.com/c/33024
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>