fuzztest: fix undefined sanitizer coverage symbol

We actually need that symbol to be exported when building with
enable_fuzztest_fuzz.

Change-Id: Iab5d843a58919a1191a83f99204c9874577458cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6281766
Commit-Queue: Paul Semel <paulsemel@chromium.org>
Reviewed-by: Adrian Taylor <adetaylor@chromium.org>
Commit-Queue: Adrian Taylor <adetaylor@chromium.org>
Auto-Submit: Paul Semel <paulsemel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1421922}
NOKEYCHECK=True
GitOrigin-RevId: 36aa4865a55686274ade2bcce23c3501100d0778