block/nbd: Add missing <qemu/bswap.h> include

block/nbd: Add missing <qemu/bswap.h> include

The inlined nbd_readXX() functions call beXX_to_cpu(), themselves
declared in <qemu/bswap.h>. This fixes when refactoring:

  In file included from ../../block/nbd.c:44:
  include/block/nbd.h: In function 'nbd_read16':
  include/block/nbd.h:383:12: error: implicit declaration of function 'be16_to_cpu' [-Werror=implicit-function-declaration]
    383 |     *val = be##bits##_to_cpu(*val);                                     \
        |            ^~
  include/block/nbd.h:387:1: note: in expansion of macro 'DEF_NBD_READ_N'
    387 | DEF_NBD_READ_N(16) /* Defines nbd_read16(). */
        | ^~~~~~~~~~~~~~

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221125175328.48539-1-philmd@linaro.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>