xen/arm: remove duplicated include from enlighten.c

Remove duplicated include.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>

arch/arm/xen/enlighten.c

@@ -10,7 +10,6 @@
 #include <xen/platform_pci.h>
 #include <xen/xenbus.h>
 #include <xen/page.h>
-#include <xen/xen.h>
 #include <xen/interface/sched.h>
 #include <xen/xen-ops.h>
 #include <asm/xen/hypervisor.h>