configure.ac: fix <netinet/tcp_var.h> detection on OpenBSD

[Austin English]

<netinet/tcp_var.h> depends on <sys/timeout.h> on OpenBSD to define
t_timer, t_delack_to, and t_reap_to.

-- 
-Austin
-------------- next part --------------
diff --git a/configure.ac b/configure.ac
index a420de2..ff82bb3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -420,6 +421,9 @@ AC_CHECK_HEADERS([netinet/tcp_timer.h netinet/udp_var.h netinet/icmp_var.h netin
      #ifdef HAVE_SYS_SOCKETVAR_H
      # include <sys/socketvar.h>
      #endif
+     #ifdef HAVE_SYS_TIMEOUT_H
+     # include <sys/timeout.h>
+     #endif
      #ifdef HAVE_NETINET_IN_H
      # include <netinet/in.h>
      #endif