tools/configurator: fix compile error with -O2

tools/configurator: fix compile error with -O2

In file included from /usr/include/string.h:495,
                 from configuratortest.c:2:
In function ‘strncpy’,
    inlined from ‘main’ at configuratortest.c:6:2:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: warning: ‘__builtin_strncpy’ specified bound 8 equals destination size [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>