qemu-iotests: Add dependency to qemu-nbd tool

qemu-iotests: Add dependency to qemu-nbd tool

Since a9660664fde, some iotests use qemu-nbd.
Add a dependency to build it before using it.
This fixes:

  $ make check-block
    GEN     qemu-img-cmds.h
    CC      qemu-img.o
    LINK    qemu-img
    CC      qemu-io.o
    LINK    qemu-io
    CC      tests/qemu-iotests/socket_scm_helper.o
    LINK    tests/qemu-iotests/socket_scm_helper
  tests/qemu-iotests-quick.sh
  check: qemu-nbd not found
  make: *** [tests/Makefile.include:1059: check-tests/qemu-iotests-quick.sh] Error 1

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>