Fix build on 32-bit with 64-bit time_t

[cgzones]

gcc reports:

../src/netlog/netlog-protocol.c: In function ‘format_rfc3339_timestamp’:
../src/netlog/netlog-protocol.c:72:62: error: format ‘%ld’ expects argument of type ‘long int’, but argument 4 has type ‘__suseconds64_t’ {aka ‘long long int’} [-Werror=format=]
   72 |                 r = snprintf(header_time, header_size, ".%06ld", tv->tv_usec);
      |                                                          ~~~~^   ~~~~~~~~~~~
      |                                                              |     |
      |                                                              |     __suseconds64_t {aka long long int}
      |                                                              long int
      |                                                          %06lld