[Fuchsia] Fix libmagma dependency

It should only be used on executable targets,
because we want to be able to choose between different
implementations.

Change-Id: I832f97c37e2acbe2e50ff70cc78fcd0e90721ad0
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/mesa/+/375393
Reviewed-by: John Bauman <jbauman@google.com>
3 files changed