MDL-76446 user: correct logic for multiple keyword search.

When the keyword participant filter was used with multiple values
in conjunction with other participants filters, the boolean logic
was effectively:

 X AND Y OR Z

When what we actually wanted was:

 X AND (Y OR Z)

user/classes/table/participants_search.php

@@ -285,7 +285,7 @@ class participants_search {
             ] = $this->get_keywords_search_sql($mappings);
 
             if (!empty($keywordswhere)) {
-                $wheres[] = $keywordswhere;
+                $wheres[] = "({$keywordswhere})";
             }
 
             if (!empty($keywordsparams)) {