perf evswitch: Add the names of on/off events

So that we can have macros for the OPT_ entries and also for finding
those in an evlist, this way other tools will use this very easily.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: William Cohen <wcohen@redhat.com>
Link: https://lkml.kernel.org/n/tip-q0og1xoqqi0w38ve5u0a43k2@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evswitch.h

@@ -9,6 +9,7 @@ struct evsel;
 
 struct evswitch {
 	struct evsel *on, *off;
+	const char   *on_name, *off_name;
 	bool	     discarding;
 	bool	     show_on_off_events;
 };