uorb: add uorb_unadvertise method

This is necessary when using multiple instances of a topic. However it does
not free the underlying resources, as it is assumed they will be used again
at a later point.

src/modules/uORB/Publication.cpp

@@ -96,6 +96,7 @@ void PublicationBase::update(void *data)
 
 PublicationBase::~PublicationBase()
 {
+	orb_unadvertise(getHandle());
 }
 
 PublicationNode::PublicationNode(const struct orb_metadata *meta,