[NETLINK]: add nla_for_each_nested() to the interface list

At the top of include/net/netlink.h is a list of Netlink interfaces, however,
the nla_for_each_nested() macro was not listed.  This patch adds this interface
to the list at the top of the header file.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>