Fix viewer build after build target renames

Commit e55567fceb5f51d81661c3ba2ba0b69a9b049762 renamed the previous
default target in sksg to legacy_sksg. Apply that change to the viewer
Bazel build file.

Change-Id: I3e75a33ba9aadb03757b7832fa111bb79dc771c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/792756
Auto-Submit: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
1 file changed