This fixes the build at least on FreeBSD, where those types were not
defined indirectly:

src/openssl_stream.c:100:18: error: variable has incomplete type 'struct in6_addr'
        struct in6_addr addr6;
                        ^
src/openssl_stream.c:100:9: note: forward declaration of 'struct in6_addr'
        struct in6_addr addr6;
               ^
src/openssl_stream.c:111:18: error: use of undeclared identifier 'AF_INET'
        if (p_inet_pton(AF_INET, host, &addr4)) {
                        ^
src/unix/posix.h:31:40: note: expanded from macro 'p_inet_pton'
                                       ^
src/openssl_stream.c:115:18: error: use of undeclared identifier 'AF_INET6'
                if(p_inet_pton(AF_INET6, host, &addr6)) {
                               ^
src/unix/posix.h:31:40: note: expanded from macro 'p_inet_pton'
                                       ^