[build] Remove obsolete //zircon/public/lib/zx label

Use //zircon/system/ulib/zx instead, which is what the alias
resolves to these days.

Bug: 42149669
Change-Id: I487c788576191b14148a00ebc29a6a947df4c499
diff --git a/BUILD.gn b/BUILD.gn
index 9518ae9..ba0a0af 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -238,7 +238,7 @@
   "$magma_build_root/src/magma_util/platform:thread",
   "$magma_build_root/src/magma_util/platform:trace",
   "$msd_build_root/msd-img-rgx/include",
-  "//zircon/public/lib/zx",
+  "//zircon/system/ulib/zx",
 ]
 
 config("mt8167_driver_config") {