perf evsel: Introduce event fallback method

The only fallback right now is for HW cpu-cycles -> SW cpu-clock, that
was done in the same way in both 'top' and 'record'.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-58l1mgibh9oa9m0pd3fasxa5@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.h

@@ -251,4 +251,7 @@ struct perf_attr_details {
 
 int perf_evsel__fprintf(struct perf_evsel *evsel,
 			struct perf_attr_details *details, FILE *fp);
+
+bool perf_evsel__fallback(struct perf_evsel *evsel, int err,
+			  char *msg, size_t msgsize);
 #endif /* __PERF_EVSEL_H */