package/xen: add dependency on host-acpica

This commit adds a dependency on the host ACPICA package as Xen tools
require iasl to build.

Fixes:

  http://autobuild.buildroot.net/results/afa199864d6b546fe759bb582a9c10702ea7fa78/



Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Erico Nunes <nunes.erico@gmail.com>
[Thomas: tweak commit log, add autobuilder reference.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>