Fix uninitialized constant with HamlLint::LinterRegistry

The migration pod for the GitLab Helm Chart was failing because
of the NameError:

uninitialized constant HamlLint::Linter::NoPlainNodes::LinterRegistry

We now explicitly specify ::HamlLint::LinterRegistry to avoid resolution
errors.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62125

lib/haml_lint/inline_javascript.rb

@@ -7,7 +7,7 @@ unless Rails.env.production?
 
   module HamlLint
     class Linter::InlineJavaScript < Linter
-      include LinterRegistry
+      include ::HamlLint::LinterRegistry
 
       def visit_filter(node)
         return unless node.filter_type == 'javascript'

lib/haml_lint/linter/no_plain_nodes.rb

@@ -5,7 +5,7 @@ require 'active_support/core_ext/array/grouping'
 module HamlLint
   class Linter
     class NoPlainNodes < Linter
-      include LinterRegistry
+      include ::HamlLint::LinterRegistry
 
       def visit_tag(node)
         if inline_plain_node?(node)