build: add sys/socket.h to dnsbl_filter

Fix building on OpenBSD.

exch/dnsbl_filter.cpp:38:16: error: use of undeclared identifier 'AF_INET6'
        if (inet_pton(AF_INET6, src, &dst) != 1) {
                      ^
1 error generated.

exch/dnsbl_filter.cpp

@@ -7,8 +7,9 @@
 #include <cerrno>
 #include <cstring>
 #include <string>
-# include <arpa/inet.h>
+#include <arpa/inet.h>
 #include <libHX/string.h>
+#include <sys/socket.h>
 #include <netinet/in.h>
 #include <gromox/config_file.hpp>
 #include <gromox/scope.hpp>