util: Always specify default visibility on exports. This fixes undefined behaviour with CFI. Bug: chromium:1015810 Bug: angleproject:3162 Change-Id: I58cfb78adabbff05e5b4560dfd70b190411fa26d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1869303 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>