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

[Austin English]

netinet/in_pcb.h depends on net/route.h on OpenBSD.

-- 
-Austin
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diff --git a/configure.ac b/configure.ac
index a420de2..d41eb89 100644
--- a/configure.ac
+++ b/configure.ac
@@ -399,6 +399,9 @@ AC_CHECK_HEADERS(\
      #ifdef HAVE_SYS_SOCKETVAR_H
      # include <sys/socketvar.h>
      #endif
+     #ifdef HAVE_NET_ROUTE_H
+     # include <net/route.h>
+     #endif
      #ifdef HAVE_NETINET_IN_H
      # include <netinet/in.h>
      #endif