acinclude: check for IP_CT_UNTRACKED

IP_CT_UNTRACKED is fairly new, but used by the kernel datapath ct_clear
action.

Signed-off-by: Eric Garver <e@erig.me>
Acked-by: Pravin B Shelar <pshelar@ovn.org>

acinclude.m4

@@ -793,6 +793,8 @@ AC_DEFUN([OVS_CHECK_LINUX_COMPAT], [
   OVS_FIND_FIELD_IFELSE([$KSRC/include/linux/netfilter.h], [nf_hook_ops],
                         [list],
                         [OVS_DEFINE([HAVE_LIST_IN_NF_HOOK_OPS])])
+  OVS_GREP_IFELSE([$KSRC/include/uapi/linux/netfilter/nf_conntrack_common.h],
+                  [IP_CT_UNTRACKED])
 
   if cmp -s datapath/linux/kcompat.h.new \
             datapath/linux/kcompat.h >/dev/null 2>&1; then