Implement hook_hook_info() to allow farm_constraint hooks to be put into [modulename].farm_constraint.inc.

modules/farm/farm_constraint/farm_constraint.module

@@ -5,6 +5,16 @@
  * Farm constraint module.
  */
 
+/**
+ * Implements hook_hook_info().
+ */
+function farm_constraint_hook_info() {
+  $hooks['farm_constraint'] = array(
+    'group' => 'farm_constraint',
+  );
+  return $hooks;
+}
+
 /**
  * Function for checking if a constraint exists (if a record is referenced by
  * other records).