arch/x86/xen/suspend.c: include xen/xen.h

Fix the build warning:

  arch/x86/xen/suspend.c: In function 'xen_arch_pre_suspend':
  arch/x86/xen/suspend.c:70:9: error: implicit declaration of function 'xen_pv_domain' [-Werror=implicit-function-declaration]
          if (xen_pv_domain())
              ^

Reported-by: kbuild test robot <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: David Vrabel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

arch/x86/xen/suspend.c

@@ -1,6 +1,7 @@
 #include <linux/types.h>
 #include <linux/tick.h>
 
+#include <xen/xen.h>
 #include <xen/interface/xen.h>
 #include <xen/grant_table.h>
 #include <xen/events.h>