xen/arm: Add asm/domain.h include to kernel.h

The 'enum domain_type' is defined by 'asm/domain.h' which is not
included (directly or indirectly) by 'asm/kernel.h'.

This currently doesn't break the compilation because asm/domain.h will
included by the user of 'kernel.h'. But it would be better to avoid
relying on it. So add the include in 'asm/domain.h'.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>

xen/arch/arm/include/asm/kernel.h

@@ -7,6 +7,7 @@
 #define __ARCH_ARM_KERNEL_H__
 
 #include <xen/device_tree.h>
+#include <asm/domain.h>
 #include <asm/setup.h>
 
 /*