[fuchsia/llvmpipe] Add missing dep to BUILD.gn

This fixes a race condition when building Lavapipe.  I suppose this
would also be a build failure on some non-Lavapipe target that
depends on llvmpipe.

Change-Id: I34a064d01aedbf1bfae1f16526a0043a538e6a07
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/1056602
Reviewed-by: Craig Stout <cstout@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Fuchsia-Auto-Submit: Josh Gargus <jjosh@google.com>
1 file changed