Several small RDF related fixes.

* [RDF_GRAPH,EXT] Allow a single graph as input argument for rdf_graph/2.
* [RDF_GRAPH,FIX] Added missing module import.
* [RDF_GRAPH,SC] Removed superfluous rdf_meta/1 declarations.
* [RDF_SERIAL,FIX] Added missing module import.
* [VOID,FIX] Using deprecated predicate for copying triples (now uses graph merging).