x86: Remove unused ACPI include

The prep_c.c source doesn't depend on the ACPI library.  As ACPI support
now requires the ACPICA module, the extra header breaks projects using
x86 without ACPI support.

Signed-off-by: Keith Short <keithshort@google.com>

arch/x86/core/prep_c.c

@@ -6,7 +6,6 @@
 
 #include <zephyr/kernel.h>
 #include <kernel_internal.h>
-#include <zephyr/acpi/acpi.h>
 #include <zephyr/arch/x86/multiboot.h>
 #include <zephyr/arch/x86/efi.h>
 #include <x86_mmu.h>