fi-0168: Client/server end must have protocol constraint {:#fi-0168}

The first constraint applied to either client_end or server_end must point to a protocol definition:

{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0168.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}

Add a constraint pointing to the desired protocol:

{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}

{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0168.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}