x86/xen: Fix kerneldoc warning

x86/xen: Fix kerneldoc warning

Fix the following W=1 kernel warnings:

arch/x86/xen/setup.c:725: warning: expecting prototype for
machine_specific_memory_setup(). Prototype was for xen_memory_setup()
instead.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20220307062554.8334-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>