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/uORB.h

@@ -142,6 +142,11 @@ extern orb_advert_t orb_advertise(const struct orb_metadata *meta, const void *d
 extern orb_advert_t orb_advertise_multi(const struct orb_metadata *meta, const void *data, int *instance,
 					int priority) __EXPORT;
 
+/**
+ * @see uORB::Manager::orb_unadvertise()
+ */
+extern int orb_unadvertise(orb_advert_t handle) __EXPORT;
+
 /**
  * Advertise as the publisher of a topic.
  *