perf ftrace: Trace system wide if no target is given

This align ftrace to other perf sub-commands that if no target specified
then we trace all functions.
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200510150628.16610-2-changbin.du@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -494,7 +494,7 @@ int cmd_ftrace(int argc, const char **argv)
 	argc = parse_options(argc, argv, ftrace_options, ftrace_usage,
 			    PARSE_OPT_STOP_AT_NON_OPTION);
 	if (!argc && target__none(&ftrace.target))
-		usage_with_options(ftrace_usage, ftrace_options);
+		ftrace.target.system_wide = true;
 
 	ret = target__validate(&ftrace.target);
 	if (ret) {