virtio-net: Fix VirtIONet typedef redefinition

Commit 2974e916 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: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NThomas Huth <thuth@redhat.com>

include/hw/virtio/virtio-net.h

@@ -106,7 +106,6 @@ typedef struct VirtioNetRscSeg {
     NetClientState *nc;
 } VirtioNetRscSeg;
 
-struct VirtIONet;
 typedef struct VirtIONet VirtIONet;
 
 /* Chain is divided by protocol(ipv4/v6) and NetClientInfo */
@@ -136,7 +135,7 @@ typedef struct VirtIONetQueue {
     struct VirtIONet *n;
 } VirtIONetQueue;
 
-typedef struct VirtIONet {
+struct VirtIONet {
     VirtIODevice parent_obj;
     uint8_t mac[ETH_ALEN];
     uint16_t status;
@@ -186,7 +185,7 @@ typedef struct VirtIONet {
     int announce_counter;
     bool needs_vnet_hdr_swap;
     bool mtu_bypass_backend;
-} VirtIONet;
+};
 
 void virtio_net_set_netclient_name(VirtIONet *n, const char *name,
                                    const char *type);