perf report: Add an indication of what time slices are used

Add a time slices indication to the perf report header.

For example,

# perf report --stdio --time 10%

# Total Lost Samples: 0
#
# Samples: 9K of event 'cycles:ppp' (time slices: 10%)
# Event count (approx.): 8951288803

Signed-off-by: Jin Yao <[e-mail]>
Suggested--by: Arnaldo Carvalho de Melo <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Reviewed-by: Jiri Olsa <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-report.c

···
 	if (evname != NULL)
 		ret += fprintf(fp, " of event '%s'", evname);
 
+	if (rep->time_str)
+		ret += fprintf(fp, " (time slices: %s)", rep->time_str);
+
 	if (symbol_conf.show_ref_callgraph &&
 	    strstr(evname, "call-graph=no")) {
 		ret += fprintf(fp, ", show reference callgraph");