perf help: Use pr_warning()

Complete the switch to using te pr_{warning,error,etc} error reporting
facilities.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-3l9gr6237b4aqyo0rsspixe2@git.kernel.orgSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-help.c

@@ -259,7 +259,7 @@ static int add_man_viewer_info(const char *var, const char *value)
 		return add_man_viewer_cmd(name, subkey - name, value);
 	}
 
-	warning("'%s': unsupported man viewer sub key.", subkey);
+	pr_warning("'%s': unsupported man viewer sub key.", subkey);
 	return 0;
 }
 
@@ -347,7 +347,7 @@ static void exec_viewer(const char *name, const char *page)
 	else if (info)
 		exec_man_cmd(info, page);
 	else
-		warning("'%s': unknown man viewer.", name);
+		pr_warning("'%s': unknown man viewer.", name);
 }
 
 static int show_man_page(const char *perf_cmd)