Fix gcc-10 warnings for packed struct members

transport.c: In function ‘booth_tcp_open’:
transport.c:656:42: warning: taking address of packed member of ‘struct booth_site’ may result in an unaligned pointer value [-Waddress-of-packed-member]

656 |  rv = connect_nonb(s, (struct sockaddr *)&to->sa6, to->saddrlen, 10);
    |                                          ^~~~~~~~

transport.c: In function ‘message_recv’:
transport.c:1108:7: warning: taking address of packed member of ‘struct booth_site’ may result in an unaligned pointer value [-Waddress-of-packed-member]
1108 | time(&source->last_recv);

|       ^~~~~~~~~~~~~~~~~~

main.c: In function ‘format_peers’:
main.c:230:14: warning: taking address of packed member of ‘struct booth_site’ may result in an unaligned pointer value [-Waddress-of-packed-member]

230 |    localtime(&s->last_recv));
    |              ^~~~~~~~~~~~~

attr.c: In function ‘store_geo_attr’:
attr.c:282:13: warning: taking address of packed member of ‘struct geo_attr’ may result in an unaligned pointer value [-Waddress-of-packed-member]

282 |    get_time(&a->update_ts);
    |             ^~~~~~~~~~~~~

timer.h:43:48: note: in definition of macro ‘get_time’

43 | #define get_time(p) clock_gettime(BOOTH_CLOCK, p)
   |                                                ^

attr.c: In function ‘append_attr’:
attr.c:337:18: warning: taking address of packed member of ‘struct geo_attr’ may result in an unaligned pointer value [-Waddress-of-packed-member]

337 |  if (is_time_set(&a->update_ts)) {
    |                  ^~~~~~~~~~~~~

attr.c:338:16: warning: taking address of packed member of ‘struct geo_attr’ may result in an unaligned pointer value [-Waddress-of-packed-member]

338 |   ts = wall_ts(&a->update_ts);
    |                ^~~~~~~~~~~~~