BUILD: connection: fix build breakage on openbsd due to missing in_systm.h

Recent commit 93b227d ("MINOR: listener: add the "accept-netscaler-cip"
option to the "bind" keyword") introduced an include of netinet/ip.h
which requires in_systm.h on OpenBSD. No backport is needed.
diff --git a/include/types/connection.h b/include/types/connection.h
index 2d10704..beb9b89 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -32,6 +32,7 @@
 #include <types/port_range.h>
 #include <types/protocol.h>
 
+#include <netinet/in_systm.h>
 #include <netinet/ip.h>
 #include <netinet/ip6.h>