Add missing frozen string literal comments

lib/gitlab/ci/external/file/base.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module External

lib/gitlab/ci/external/file/local.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module External

lib/gitlab/ci/external/file/remote.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module External

lib/gitlab/ci/external/mapper.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module External

lib/gitlab/ci/external/processor.rb

+# frozen_string_literal: true
+
 module Gitlab
   module Ci
     module External

spec/lib/gitlab/ci/external/file/local_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Ci::External::File::Local do

spec/lib/gitlab/ci/external/file/remote_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Ci::External::File::Remote do

spec/lib/gitlab/ci/external/mapper_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Ci::External::Mapper do

spec/lib/gitlab/ci/external/processor_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe Gitlab::Ci::External::Processor do