arm64: ipi_nmi: fix compile error when CONFIG_KGDB is disabled

hulk inclusion
category: bugfix
bugzilla: 50485
CVE: NA

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Fix the following compile error.

arch/arm64/kernel/ipi_nmi.c: In function ‘ipi_nmi_handler’:
arch/arm64/kernel/ipi_nmi.c:54:7: error: implicit declaration of function ‘kgdb_nmicallback’ [-Werror=implicit-function-declaration]
  if (!kgdb_nmicallback(cpu, get_irq_regs()))
       ^~~~~~~~~~~~~~~~

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>