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_unit':
  util/header.c:3586:2: error: 'strncpy' output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
    strncpy(ev->data, evsel->unit, size);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  util/header.c:3579:16: note: length computed here
    size_t size = strlen(evsel->unit);
                  ^~~~~~~~~~~~~~~~~~~

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Fixes: a6e52817 ("perf tools: Add event_update event unit type")
Link: https://lkml.kernel.org/n/tip-fiikh5nay70bv4zskw2aa858@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>