xen/arm: compile netback

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 682633bf..1c0a302 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -40,6 +40,7 @@
 
 #include <net/tcp.h>
 
+#include <xen/xen.h>
 #include <xen/events.h>
 #include <xen/interface/memory.h>