[build][alln] Fix Zircon library dependency

This CL assumes [1] has been submitted to fuchsia.git. It changes
references to the zircon library to a proper target dependency, as
required to move the sysroot generation to the Fuchsia build. See
[1] for more details.

Bug: 60613
Testing: Should not change the build.

[1] https://fuchsia-review.googlesource.com/c/fuchsia/+/433557

Change-Id: I519e8f30105f47b62cc334797a735cd36d99390d