Fix compilation warning

ovirt-resource-rest-call.c: In function 'ovirt_resource_rest_call_class_serialize_params':
ovirt-resource-rest-call.c:141:5: error: passing argument 2 of 'ovirt_resource_add_rest_params' from incompatible pointer type [-Werror]
     ovirt_resource_add_rest_params(self->priv->resource, self);
     ^
In file included from ovirt-resource-rest-call.c:32:0:
ovirt-resource-private.h:51:6: note: expected 'struct RestProxyCall *' but argument is of type 'struct OvirtResourceRestCall *'
 void ovirt_resource_add_rest_params(OvirtResource *resource,
      ^
cc1: all warnings being treated as errors