libsystemd-network: remove _packed_ attribute to appease the compiler

libsystemd-network: remove _packed_ attribute to appease the compiler

gcc-9 warns whenever the elements of a structure defined with _packed_ are used:

../src/network/networkd-dhcp6.c: In function ‘dhcp6_pd_prefix_assign’:
../src/network/networkd-dhcp6.c:92:53: warning: taking address of packed member of ‘struct <anonymous>’ may result in an unaligned pointer value [-Waddress-of-packed-member]
   92 |         r = manager_dhcp6_prefix_add(link->manager, &p->opt.in6_addr, link);
      |                                                     ^~~~~~~~~~~~~~~~

And the compiler is right, because in principle the alignment could be wrong.
In this particular case it is not, because the structure is carefully defined
not to have holes. Let's remove _packed_ and use compile-time asserts to verify
that the offsets are not changed.