Update path to mock magma_system

It was recently moved to //src/graphics/magma/lib/magma_client/mock:magma_system.

Bug: 133551
Change-Id: I0a31a4825d81c2b838965edb6ebb5dd0d7e31d0b
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/922773
Fuchsia-Auto-Submit: John Bauman <jbauman@google.com>
Reviewed-by: Craig Stout <cstout@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
1 file changed