libxl: include sys/ioctl.h

libxl: include sys/ioctl.h

This is a requirement for the code in libxl_remus_disk_drbd.c that uses the
ioctl syscall. Fixes the following error when building on FreeBSD:

libxl_remus_disk_drbd.c: In function 'drbd_postsuspend':
libxl_remus_disk_drbd.c:197:9: error: implicit declaration of function 'ioctl' [-Werror=implicit-function-declaration]

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: Yang Hongyang <yanghy@cn.fujitsu.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>