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/probe_64.c

@@ -9,7 +9,8 @@
  * James Cleverdon.
  */
 #include <asm/apic.h>
-#include <asm/ipi.h>
+
+#include "ipi.h"
 
 /*
  * Check the APIC IDs in bios_cpu_apicid and choose the APIC mode.