x86: fix section mismatch warning - apic_physflat

Impact: cleanup only, no functionality changed

WARNING: vmlinux.o(.data+0xbe88): Section mismatch in reference from the variable apic_physflat to the function .init.text:physflat_acpi_madt_oem_check()
The variable apic_physflat references
the function __init physflat_acpi_madt_oem_check()

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