[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: NPaul Moore <paul.moore@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -151,6 +151,7 @@
  *   nla_parse()			parse and validate stream of attrs
  *   nla_parse_nested()			parse nested attribuets
  *   nla_for_each_attr()		loop over all attributes
+ *   nla_for_each_nested()		loop over the nested attributes
  *=========================================================================
  */