x86/apic: Move apic_flat_64 header into apic directory

Only used locally.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190722105219.526508168@linutronix.de