This happened to be working only because of lucky -I argument and At the
same time, include digest.h since this file references |EVP_sha1| and
other digest-related functions.

Change-Id: I0095ea8f5ef21f6e63b3dc819932b38178e09693
Reviewed-on: https://boringssl-review.googlesource.com/28244
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>