mm: init: Fix build waring for ascend enable features

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I66EW4


CVE: NA

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When enable Ascend enable CONFIG_ASCEND_FEATURES, and enable
CONFIG_ASCEND_CHARGE_MIGRATE_HUGEPAGES, and build kernel image,
it will have build warning as follows:

arch/arm64/mm/init.c:736:2: warning: ISO C90 fobids mixed
declarations and code [-Wdeclaration-atfer-statement]
  736 | extern int enable_charge_mighp;

Signed-off-by: Zhang Jian <zhangjian210@huawei.com>
Reviewed-by: chenweilong <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>