[cobalt] Fix BUILD.gn to not expect fuchsia gni files

This is similar to previous change f3191da which was undone by 16e9935,
preventing Cobalt from updating to the latest boringssl version. We now
need to update in order to get the recent conversion warning changes
done by shayba@.

BUG: 55352
BUG: 59275
Change-Id: Ie4d931106873d9cb56dd3907f1f2ceb9e2fd1a33
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/boringssl/+/430717
Reviewed-by: Zach Bush <zmbush@google.com>
Commit-Queue: Cameron Dale <camrdale@google.com>
1 file changed