include/uipc: Add "topic" field to pubsub UIPC messages

include/uipc: Add "topic" field to pubsub UIPC messages

Messages that were sent with uipc_endpoint_publish() don't contain
information about the topic that they were published on, making it
impossible for the receiver to determine what topic a message was
published on if they are subscribed to more than one topic.

This commit adds a new field "topic" to UIPC messages and modifies
uipc_endpoint_publish() so that it sets the field appropriately. This
further adds the function uipc_msg_get_topic() that may be used to
determine the topic a UIPC message was published on.