x86, acpi/irq: Define gsi_end when X86_IO_APIC is undefined

My recent changes introducing a global gsi_end variable
failed to take into account the case of using acpi on a system
not built to support IO_APICs, causing the build to fail.

Define gsi_end to 15 when CONFIG_X86_IO_APIC is not set to avoid
compile errors.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <m1tyqm14la.fsf_-_@fess.ebiederm.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>