mvc: translate base field error

Without gettext() wrapping we never produce a string in the
translation for it.  It gets passed to gettext() twice then
but that is matters not as the target string cannot be translated
again when called a second time.

src/opnsense/mvc/app/models/OPNsense/Base/FieldTypes/BaseListField.php

@@ -68,7 +68,7 @@ abstract class BaseListField extends BaseField
     protected function getValidationMessage()
     {
         if ($this->internalValidationMessage == null) {
-            return "option not in list";
+            return gettext('option not in list');
         } else {
             return $this->internalValidationMessage;
         }