xen/arm: Fix compilation error when early printk is enabled

xen/arm: Fix compilation error when early printk is enabled

Fix compilation error when enabling early printk, introduced
by commit aa4b9d1ee6538b5cbe218d4d3fcdf9548130a063:
```
debug.S: Assembler messages:
debug.S:31: Error: constant expression expected at operand 2 -- `ldr x15,=((0x00400000+(0)*PAGE_SIZE)+(0x1c090000&~PAGE_MASK))`
debug.S:38: Error: constant expression expected at operand 2 -- `ldr x15,=((0x00400000+(0)*PAGE_SIZE)+(0x1c090000&~PAGE_MASK))`
```

The fix is to include header <xen/page-size.h> which now contains
definitions for page/size/mask etc.

Fixes: aa4b9d1ee653 ("include: don't use asm/page.h from common headers")
Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>