libxl: fix libacpi dependency

libxl: fix libacpi dependency

$(DSDT_FILES-y) depends on the recursive make to have run in libacpi/
such that the file(s) itself/themselves were generated before
compilation gets attempted. The same, however, is also necessary for
generated headers, before source files including them would get
attempted to be compiled.

The dependency specified in libacpi's Makefile, otoh, is entirely
pointless nowadays - no compilation happens there anymore (except for
tools involved in building the generated files). Together with it, the
rule generating acpi.a also can go away.

Reported-by: Olaf Hering <olaf@aepfle.de>
Fixes: 14c0d328da2b ("libxl/acpi: Build ACPI tables for HVMlite guests")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wl@xen.org>