Declare $_subfields property

* src/HAB/Pica/Record/Field.php: Declare $_subfields property.

...after all this years... ;)

src/HAB/Pica/Record/Field.php

@@ -132,6 +132,13 @@ class Field
      */
     protected $_shorthand;
 
+    /**
+     * Subfields.
+     *
+     * @var array
+     */
+    protected $_subfields;
+
     /**
      * Constructor.
      *