static-evtchn: Add missing include

When later on x86 starts using this file in later patches it won't find
device_tree.h because it's only transitively included by arm.

Make it explicit.

Not a functional change.

Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
Acked-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>

xen/common/device-tree/static-evtchn.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
 #include <xen/bootfdt.h>
+#include <xen/device_tree.h>
 #include <xen/event.h>
 #include <xen/static-evtchn.h>