Convert third_party source sets to static libraries.

third_party/libsecret/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file
 
 config("libsecret_config") {
   include_dirs = [ rebase_path("./") ]
 }
 
-source_set("libsecret") {
+static_library("libsecret") {
   sources = [
     "libsecret/secret-attributes.h",
     "libsecret/secret-collection.h",
     "libsecret/secret-enum-types.h",
     "libsecret/secret-item.h",
     "libsecret/secret-password.h",
     "libsecret/secret-paths.h",
     "libsecret/secret-prompt.h",
     "libsecret/secret-schema.h",
     "libsecret/secret-schemas.h",
     "libsecret/secret-service.h",
     "libsecret/secret-types.h",
     "libsecret/secret-value.h",
     "libsecret/secret.h",
   ]
 
   public_configs = [ ":libsecret_config" ]
 }