intel/decoder: use snprintf(..., "%s", ...) instead of strncpy

intel/decoder: use snprintf(..., "%s", ...) instead of strncpy

strncpy() doesn't guarantee the terminator NUL, so we would need to
set ourselves. Just use snprintf() instead.

Fixes the warnings

../../src/intel/common/gen_decoder.c: In function ‘iter_decode_field’:
../../src/intel/common/gen_decoder.c:897:7: warning: ‘strncpy’ specified bound 128 equals destination size [-Wstringop-truncation]
       strncpy(iter->name, iter->field->name, sizeof(iter->name));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In function ‘iter_advance_field’,
    inlined from ‘gen_field_iterator_next’ at ../../src/intel/common/gen_decoder.c:1015:9:
../../src/intel/common/gen_decoder.c:844:7: warning: ‘strncpy’ specified bound 128 equals destination size [-Wstringop-truncation]
       strncpy(iter->name, iter->field->name, sizeof(iter->name));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>