Add missing enum member force-forwarding to ipv6-devconf.

In commit f24987ef6959 ("ipv6: add `force_forwarding` sysctl to
enable per-interface forwarding"), the DEVCONF_* enum was extended
with DEVCONF_FORCE_FORWARDING, however Gabriel forgot to add it
to the spec.

This pre-existing issue was detected by Sashiko, when reviewing the
first patch in this series.

Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260824103816.40040-1-ast%40fiberby.net
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
---
 Documentation/netlink/specs/rt-link.yaml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/netlink/specs/rt-link.yaml b/Documentation/netlink/specs/rt-link.yaml
index ab677801cde8..61ebb9a2bad5 100644
--- a/Documentation/netlink/specs/rt-link.yaml
+++ b/Documentation/netlink/specs/rt-link.yaml
@@ -496,6 +496,8 @@ definitions:
         name: accept-untracked-na
       -
         name: accept-ra-min-lft
+      -
+        name: force-forwarding
   -
     name: ifla-icmp6-stats
     enum-name:
-- 
2.55.0