BGP: Address a -Wformat-truncation= compiler warning

BGP: Address a -Wformat-truncation= compiler warning

The warning was:
./print-bgp.c:973:79: warning: 'snprintf' output may be truncated before the last format character [-Wformat-truncation=]
  973 |         snprintf(output, sizeof(output), "route-target: %u.%u.%u.%u:%u/%d (%s)",
      |                                                                               ^
./print-bgp.c:973:9: note: 'snprintf' output between 30 and 61 bytes into a destination of size 60
  973 |         snprintf(output, sizeof(output), "route-target: %u.%u.%u.%u:%u/%d (%s)",
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  974 |                  *(pptr+2), *(pptr+3), *(pptr+4), *(pptr+5),
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  975 |                  EXTRACT_BE_U_2(pptr+6), plen, rtc_prefix_in_hex);
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~