Silence debug notices by manually defining $groupby.

This change is necessary because the WP_Signup_Query class follows the common pattern used by WordPress itself, which does have support for MySQL GROUP BY.

Author: JJJ

wp-user-signups/includes/classes/class-wp-signup-query.php

@@ -561,6 +561,9 @@ protected function get_signup_ids() {
 
 		$where = implode( ' AND ', $this->sql_clauses['where'] );
 
+		// Not currently used. Set to empty to prevent debug notices.
+		$groupby = '';
+
 		$pieces = array( 'fields', 'join', 'where', 'orderby', 'limits', 'groupby' );
 
 		/**