virtio-net: Fix VirtIONet typedef redefinition

virtio-net: Fix VirtIONet typedef redefinition

Commit 2974e916df8 introduced the VirtioNetRscChain structure which
refer to a VirtIONet, declared later, thus required VirtIONet typedef
to use a forward declaration.
However, when compiling with Clang in -std=gnu99 mode, this triggers
the following warning/error:

    CC      hw/net/virtio-net.o
  In file included from qemu/hw/net/virtio-net.c:22:
  include/hw/virtio/virtio-net.h:189:3: error: redefinition of typedef 'VirtIONet' is a C11 feature [-Werror,-Wtypedef-redefinition]
  } VirtIONet;
    ^
  include/hw/virtio/virtio-net.h:110:26: note: previous definition is here
  typedef struct VirtIONet VirtIONet;
                           ^
  1 error generated.
  make: *** [rules.mak:69: hw/net/virtio-net.o] Error 1

Fix it by removing the duplicate typedef definition.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>