x86: fix section mismatch warning - apic_flat

Impact: cleanup only, no functionality changed

WARNING: vmlinux.o(.data+0xbe08): Section mismatch in reference from the variable apic_flat to the function .init.text:flat_acpi_madt_oem_check()
The variable apic_flat references
the function __init flat_acpi_madt_oem_check()

This is harmless, because the .acpi_madt_oem_check is only called
during init time. But we keep the function pointer around in a .data
function pointer template, so it's better we do not keep that stale
- so mark this function non-__init.

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