libxl: Properly declare libxlu_disk_l.h in AUTOINCS

libxl: Properly declare libxlu_disk_l.h in AUTOINCS

This is necessary so that make doesn't do things which depend on this
file until flex has finished producing it.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Olaf Hering <olaf@aepfle.de>
Tested-by: Olaf Hering <olaf@aepfle.de>
CC: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>