[fidl][cpp] Mark a public FIDL dependency appropriately

Soon this will matter. Right now it has very little effect.

FIDL-538

Change-Id: I083ab28d1bd28ac7ff1dc866be888c0d563992c4
diff --git a/auth_providers/oauth/BUILD.gn b/auth_providers/oauth/BUILD.gn
index 76f37b8..44c23c5 100644
--- a/auth_providers/oauth/BUILD.gn
+++ b/auth_providers/oauth/BUILD.gn
@@ -12,8 +12,11 @@
     "oauth_response.h",
   ]
 
-  deps = [
+  public_deps = [
     "//sdk/fidl/fuchsia.auth",
+    "//sdk/fidl/fuchsia.net.oldhttp",
+  ]
+  deps = [
     "//garnet/public/lib/fsl",
     "//garnet/public/lib/fxl",
     "//garnet/public/lib/network_wrapper",