[zircon][lib][zxtest] Add missing puiblic deps.

For fuchsia builds, zxtest stringifies zx_status_t on ASSERT/EXPECT_OK
macros. This requires exposing <zircon/status.h>.

TEST=build, CQ.

Change-Id: I386751713a2f645a5a0e15d1d485ac9f119b226f
diff --git a/zircon/system/ulib/zxtest/BUILD.gn b/zircon/system/ulib/zxtest/BUILD.gn
index b46203a..8b79ec5 100644
--- a/zircon/system/ulib/zxtest/BUILD.gn
+++ b/zircon/system/ulib/zxtest/BUILD.gn
@@ -28,10 +28,19 @@
     "zxtest-main.cpp",
   ]
   configs += [ "$zx/public/gn/config:visibility_hidden" ]
+  public_deps = [
+      # Needed for <fbl/string.h>, <fbl/vector.h> and <fbl/function.h>.
+      "$zx/system/ulib/fbl:headers",
+  ]
   deps = [
     "$zx/system/ulib/fbl",
   ]
   if (is_fuchsia) {
+    public_deps += [
+      # Needed for <zircon/status.h>
+      "$zx/system/ulib/zircon:headers",
+    ]
+
     deps += [
       "$zx/system/ulib/zircon",
       "$zx/system/ulib/zx",