perf jevents: Add __maybe_unused attribute to unused function arg

The tools/perf/pmu-events/jevents.c file isn't being compiled with
-Werror and -Wextra, which will be the case soon, so before we turn
those compiler flags on, fix what it would flag.

Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Like Xu <like.xu.linux@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
To: John Garry <john.garry@huawei.com>

tools/perf/pmu-events/jevents.c

@@ -45,6 +45,7 @@
 #include <sys/resource.h>		/* getrlimit */
 #include <ftw.h>
 #include <sys/stat.h>
+#include <linux/compiler.h>
 #include <linux/list.h>
 #include "jsmn.h"
 #include "json.h"
@@ -470,7 +471,7 @@ static void free_arch_std_events(void)
 	}
 }
 
-static int save_arch_std_events(void *data, struct json_event *je)
+static int save_arch_std_events(void *data __maybe_unused, struct json_event *je)
 {
 	struct event_struct *es;