net/vhost-vdpa: Include missing 'qemu/iov.h' header

vhost-vdpa.c uses methods declared in the "qemu/iov." header.
Include it otherwise we get when refactoring unrelated headers:

  ../net/vhost-vdpa.c: In function ‘vhost_vdpa_net_load_cmd’:
  ../net/vhost-vdpa.c:714:24: error: implicit declaration of function ‘iov_size’
    714 |     size_t data_size = iov_size(data_sg, data_num), cmd_size;
        |                        ^~~~~~~~
  ../net/vhost-vdpa.c:714:24: error: nested extern declaration of ‘iov_size’
  ../net/vhost-vdpa.c:742:5: error: implicit declaration of function ‘iov_from_buf’
    742 |     iov_from_buf(out_cursor, 1, 0, &ctrl, sizeof(ctrl));
        |     ^~~~~~~~~~~~
  ../net/vhost-vdpa.c:742:5: error: nested extern declaration of ‘iov_from_buf’
  ../net/vhost-vdpa.c:744:5: error: implicit declaration of function ‘iov_to_buf’
    744 |     iov_to_buf(data_sg, data_num, 0,
        |     ^~~~~~~~~~
  ../net/vhost-vdpa.c:744:5: error: nested extern declaration of ‘iov_to_buf’
  ../net/vhost-vdpa.c:748:5: error: implicit declaration of function ‘iov_copy’
    748 |     iov_copy(&out, 1, out_cursor, 1, 0, cmd_size);
        |     ^~~~~~~~

Fixes: bd907ae4b0 ("vdpa: manual forward CVQ buffers")
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20260812211708.92824-3-philmd@oss.qualcomm.com>

net/vhost-vdpa.c

@@ -17,6 +17,7 @@
 #include "hw/virtio/vhost-vdpa.h"
 #include "qemu/config-file.h"
 #include "qemu/error-report.h"
+#include "qemu/iov.h"
 #include "qemu/log.h"
 #include "qemu/memalign.h"
 #include "qemu/option.h"