x86: fix compile warning building without CONFIG_SYSCTL

arch/x86/kernel/nmi_64.c:50: warning: 'unknown_nmi_panic_callback' declared 'static' but never defined

This patch also fixes nmi_32.c

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>