perf data: Fix 'strncat may truncate' build failure with recent gcc

This strncat() is safe because the buffer was allocated with zalloc(),
however gcc doesn't know that. Since the string always has 4 non-null
bytes, just use memcpy() here.

    CC       /home/shawn/linux/tools/perf/util/data-convert-bt.o
  In file included from /usr/include/string.h:494,
                   from /home/shawn/linux/tools/lib/traceevent/event-parse.h:27,
                   from util/data-convert-bt.c:22:
  In function ‘strncat’,
      inlined from ‘string_set_value’ at util/data-convert-bt.c:274:4:
  /usr/include/powerpc64le-linux-gnu/bits/string_fortified.h:136:10: error: ‘__builtin_strncat’ output may be truncated copying 4 bytes from a string of length 4 [-Werror=stringop-truncation]
    136 |   return __builtin___strncat_chk (__dest, __src, __len, __bos (__dest));
        |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Shawn Landden <shawn@git.icu>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
LPU-Reference: 20190518183238.10954-1-shawn@git.icu
Link: https://lkml.kernel.org/n/tip-289f1jice17ta7tr3tstm9jm@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>