x86: fix section mismatch warning - apic_x2apic_cluster

Impact: cleanup only, no functionality changed

WARNING: vmlinux.o(.data+0xbf88): Section mismatch in reference from the variable apic_x2apic_cluster to the function .init.text:x2apic_acpi_madt_oem_check()
The variable apic_x2apic_cluster references
the function __init x2apic_acpi_madt_oem_check()

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>