[bazel] Add "gazelle" target to bazel/Makefile

This is easier for me to find, as I more often work in the infra repo.

Change-Id: I93d02299604c8b502f89576ada7597a8443b24e8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1049956
Auto-Submit: Eric Boren <borenet@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>