[build] Remove //zircon/public/lib/zx reference.

Bug: 70426
Change-Id: Idea8025c46ab218c190a9c587836fee17b6abd8c
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/kmackay/micro-ecc/+/487358
Reviewed-by: Shai Barack <shayba@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 0e0a61b..a37c15e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -22,7 +22,7 @@
   public_configs = [ ":micro-ecc_config" ]
   deps = [
     ":micro-ecc-headers",
-    "//zircon/public/lib/zx",
+    "//zircon/system/ulib/zx",
   ]
 
   # TODO(fxb/58162): delete the below and fix compiler warnings