Fix missing relative include path on utests

This change will fix an include path that was missing to allow
building zlib's unit tests outside of Chromium tree.

Bug: 1032721
Change-Id: I9b4e4a0b816e716b7e1cda9172544d02127f0d32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2242125


Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#777690}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ea5c8e29bd38461168d12eb1c6c7b5312a7c56db