xen/pvh: Fix compile issues with xen_pvh_domain()

Oddly enough it compiles for my ancient compiler but with
the supplied .config it does blow up. Fix is easy enough.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/grant-table.c

@@ -128,6 +128,7 @@ void arch_gnttab_unmap(void *shared, unsigned long nr_gframes)
 #ifdef CONFIG_XEN_PVH
 #include <xen/balloon.h>
 #include <xen/events.h>
+#include <xen/xen.h>
 #include <linux/slab.h>
 static int __init xlated_setup_gnttab_pages(void)
 {