string: Fix bug-strncat1 with fortify enabled

If fortify is enabled, the truncated output warning is issued by
the wrapper itself:

bug-strncat1.c: In function ‘main’:
bug-strncat1.c:14:3: error: ‘__builtin___strncat_chk’ output truncated
copying 1 byte from a string of length 2 [-Werror=stringop-truncation]
   14 |   strncat (d, "\5\6", 1);
      |   ^

Checked on x86_64-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>