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

arch/x86/kernel/apic/apic_flat_64.c

@@ -13,9 +13,9 @@
 #include <linux/acpi.h>
 
 #include <asm/jailhouse_para.h>
-#include <asm/apic_flat_64.h>
 #include <asm/apic.h>
 
+#include "apic_flat_64.h"
 #include "ipi.h"
 
 static struct apic apic_physflat;

arch/x86/kernel/apic/apic_numachip.c

@@ -16,9 +16,9 @@
 #include <asm/numachip/numachip.h>
 #include <asm/numachip/numachip_csr.h>
 
-#include <asm/apic_flat_64.h>
 #include <asm/pgtable.h>
 
+#include "apic_flat_64.h"
 #include "ipi.h"
 
 u8 numachip_system __read_mostly;