perf header: Fix unchecked usage of strncpy()

The strncpy() function may leave the destination string buffer
unterminated, better use strlcpy() that we have a __weak fallback
implementation for systems without it.

This fixes this warning on an Alpine Linux Edge system with gcc 8.2:

util/header.c: In function 'perf_event__synthesize_event_update_name':
util/header.c:3625:2: error: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
strncpy(ev->data, evsel->name, len);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
util/header.c:3618:15: note: length computed here
size_t len = strlen(evsel->name);
^~~~~~~~~~~~~~~~~~~

Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Fixes: a6e5281780d1 ("perf tools: Add event_update event unit type")
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/header.c

···
 	if (ev == NULL)
 		return -ENOMEM;
 
-	strncpy(ev->data, evsel->name, len);
+	strlcpy(ev->data, evsel->name, len + 1);
 	err = process(tool, (union perf_event*) ev, NULL, NULL);
 	free(ev);
 	return err;