Documentation: networking: add ioam6-sysctl into index

Append ioam6-sysctl to toctree in order to get rid of building warnings.
Signed-off-by: Hu Haowen <src.res@email.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -57,6 +57,7 @@ Contents:
    gen_stats
    gtp
    ila
+   ioam6-sysctl
    ipddp
    ip_dynaddr
    ipsec