MDL-26099 fixing the strict standards warning

The signature of mod_workshop_mod_form::validation() is now compatible
with that of moodleform_mod::validation() even though the parent's
declaration sucks. No one can choose their parents.

mod/workshop/mod_form.php

@@ -297,7 +297,7 @@ public function definition_after_data() {
      * @param array $files submitted files
      * @return array eventual errors indexed by the field name
      */
-    public function validation(array $data, array $files) {
+    public function validation($data, $files) {
         $errors = array();
 
         // check the phases borders are valid