Fix: only add components to graph if they are not already present

Fix: only add components to graph if they are not already present

bt_graph_connect() creates a connection and adds the components
on both ends of the connection to the graph. This fix ensures
that the components are not added a second time if multiple
connections involving the same components are established.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>