I18N: Add a class_exists() check to Plural_Forms class for consistency with other POMO library classes.

Follow-up to [41722].

Fixes #50881.

git-svn-id: https://develop.svn.wordpress.org/trunk@48769 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/pomo/plural-forms.php

@@ -5,6 +5,7 @@
  *
  * @since 4.9.0
  */
+if ( ! class_exists( 'Plural_Forms', false ) ) :
 	class Plural_Forms {
 		/**
 		 * Operator characters.
@@ -342,3 +343,4 @@ class Plural_Forms {
 			return (int) $stack[0];
 		}
 	}
+endif;