Use helper function to add to engaged modules

Use the existing helper function to add a module to the engaged modules list,
rather than modifying the list directly.  This handles the case where the list
has not already been initialized.

Jira: AXIS2C-1697



diff --git a/src/core/description/op.c b/src/core/description/op.c
index 900473c..f0ddbc1 100644
--- a/src/core/description/op.c
+++ b/src/core/description/op.c
@@ -672,7 +672,8 @@
         }
         /* module = axis2_module_desc_get_module(moduleref, env); */
 
-        axutil_array_list_add(collection_module, env, moduleref);
+        /*axutil_array_list_add(collection_module, env, moduleref);*/
+        status = axis2_op_add_to_engaged_module_list(op, env, moduleref);
     }
     else
     {