configure: Fix <netinet/tcp_var.h> detection on OpenBSD.
diff --git a/configure.ac b/configure.ac
index 122191d..063ea15 100644
--- a/configure.ac
+++ b/configure.ac
@@ -423,6 +423,9 @@
      #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