[Nauty] Correction to traces.h

[Mathieu Dutour]

For the traces.h I think the __cplusplus statements are missing.
Traces is performing better in some cases and one would like
to be able to access it from the C++ code.

#ifdef __cplusplus
extern "C" {
#endif

extern void Traces(sparsegraph*,int*,int*,int*,TracesOptions*,
                                   TracesStats*,sparsegraph*);
extern void refine_tr(sparsegraph*,int*,int*,int*,int*,TracesOptions*);
extern void traces_freedyn(void);

#ifdef __cplusplus
}
#endif