Add __nonstring annotation, since the missing string termination for
id member of sclp_trace_entry is intended. This way we get rid of this
warning:

drivers/s390/char/sclp.c:84:9: warning: ‘strncpy’ output truncated before terminating nul copying 4 bytes from a string of the same length [-Wstringop-truncation]
   84 |         strncpy(e.id, id, sizeof(e.id));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>