[zircon][gn] Fix legacy export of trace-handler library

This makes it possible for Fuchsia GN targets to depend
on //zircon/public/lib/trace-handler or on a static library
like //zircon/public/lib/trace-test-utils that uses it.

Bug: BLD-473 #done
Test: no immediate effect on the build
Change-Id: If29ed149c178c2aae0d03316d518de6e2d3c22a0