x86/apic: Move ipi 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/20190722105219.434738036@linutronix.de

arch/x86/kernel/apic/x2apic_phys.c

@@ -3,9 +3,8 @@
 #include <linux/cpumask.h>
 #include <linux/acpi.h>
 
-#include <asm/ipi.h>
-
 #include "x2apic.h"
+#include "ipi.h"
 
 int x2apic_phys;