x86/mm/hotplug: Put kernel_physical_mapping_remove() declaration in CONFIG_MEMORY_HOTREMOVE

kernel_physical_mapping_remove() is only called by
arch_remove_memory() in init_64.c, which is enclosed in
CONFIG_MEMORY_HOTREMOVE. So when we don't configure
CONFIG_MEMORY_HOTREMOVE, the compiler will give a warning:

	warning: ‘kernel_physical_mapping_remove’ defined but not used

So put kernel_physical_mapping_remove() in
CONFIG_MEMORY_HOTREMOVE.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Cc: linux-mm@kvack.org
Cc: gregkh@linuxfoundation.org
Cc: yinghai@kernel.org
Cc: wency@cn.fujitsu.com
Cc: mgorman@suse.de
Cc: tj@kernel.org
Cc: liwanp@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1366019207-27818-3-git-send-email-tangchen@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>