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 <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>

drivers/xen/pcpu.c

@@ -40,6 +40,7 @@
 #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>