Fix warning messages with SVG ALLOWED_ELEMENTS

Eliminates "warning: already initialized constant Gitlab::Sanitizers::SVG::ALLOWED_ELEMENTS"

lib/gitlab/sanitizers/svg.rb

-require_relative "svg/whitelist"
-
 module Gitlab
   module Sanitizers
     module SVG
@@ -12,14 +10,14 @@ module Gitlab
         DATA_ATTR_PATTERN = /\Adata-(?!xml)[a-z_][\w.\u00E0-\u00F6\u00F8-\u017F\u01DD-\u02AF-]*\z/u
 
         def scrub(node)
-          unless ALLOWED_ELEMENTS.include?(node.name)
+          unless Whitelist::ALLOWED_ELEMENTS.include?(node.name)
             node.unlink
           else
             node.attributes.each do |attr_name, attr|
-              valid_attributes = ALLOWED_ATTRIBUTES[node.name]
+              valid_attributes = Whitelist::ALLOWED_ATTRIBUTES[node.name]
 
               unless valid_attributes && valid_attributes.include?(attr_name)
-                if ALLOWED_DATA_ATTRIBUTES_IN_ELEMENTS.include?(node.name) &&
+                if Whitelist::ALLOWED_DATA_ATTRIBUTES_IN_ELEMENTS.include?(node.name) &&
                     attr_name.start_with?('data-')
                   # Arbitrary data attributes are allowed. Verify that the attribute
                   # is a valid data attribute.