drivers: xen: Include appropriate header file in pcpu.c

Include appropriate header file in xen/pcpu.c because include/xen/acpi.h
contains prototype declaration of functions defined in the file.

This eliminates the following warning in xen/pcpu.c:
drivers/xen/pcpu.c:336:6: warning: no previous prototype for ‘xen_pcpu_hotplug_sync’ [-Wmissing-prototypes]
drivers/xen/pcpu.c:346:5: warning: no previous prototype for ‘xen_pcpu_id’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <[e-mail]>
Reviewed-by: Josh Triplett <[e-mail]>
Signed-off-by: Konrad Rzeszutek Wilk <[e-mail]>
Reviewed-by: David Vrabel <[e-mail]>

drivers/xen/pcpu.c

···
 #include <linux/capability.h>
 
 #include <xen/xen.h>
+#include <xen/acpi.h>
 #include <xen/xenbus.h>
 #include <xen/events.h>
 #include <xen/interface/platform.h>