net: rtnetlink.h -- only include linux/netdevice.h when used by the kernel

The commit below added a new helper dev_ingress_queue to cleanly obtain the
ingress queue pointer.  This necessitated including 'linux/netdevice.h':

  commit 24824a09e35402b8d58dcc5be803a5ad3937bdba
  Author: Eric Dumazet <eric.dumazet@gmail.com>
  Date:   Sat Oct 2 06:11:55 2010 +0000

    net: dynamic ingress_queue allocation

However this include triggers issues for applications in userspace
which use the rtnetlink interfaces.  Commonly this requires they include
'net/if.h' and 'linux/rtnetlink.h' leading to a compiler error as below:

  In file included from /usr/include/linux/netdevice.h:28:0,
                   from /usr/include/linux/rtnetlink.h:9,
                   from t.c:2:
  /usr/include/linux/if.h:135:8: error: redefinition of ‘struct ifmap’
  /usr/include/net/if.h:112:8: note: originally defined here
  /usr/include/linux/if.h:169:8: error: redefinition of ‘struct ifreq’
  /usr/include/net/if.h:127:8: note: originally defined here
  /usr/include/linux/if.h:218:8: error: redefinition of ‘struct ifconf’
  /usr/include/net/if.h:177:8: note: originally defined here

The new helper is only defined for the kernel and protected by __KERNEL__
therefore we can simply pull the include down into the same protected
section.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>