x86: include linux/ratelimit.h in nmi.c

When building random configurations, we now occasionally get a new
build error:

   In file included from include/linux/kernel.h:13:0,
                    from include/linux/list.h:8,
                    from include/linux/preempt.h:10,
                    from include/linux/spinlock.h:50,
                    from arch/x86/kernel/nmi.c:13:
   arch/x86/kernel/nmi.c: In function 'nmi_max_handler':
   include/linux/printk.h:375:9: error: type defaults to 'int' in declaration of 'DEFINE_RATELIMIT_STATE' [-Werror=implicit-int]
     static DEFINE_RATELIMIT_STATE(_rs,    \
            ^
   arch/x86/kernel/nmi.c:110:2: note: in expansion of macro 'printk_ratelimited'
     printk_ratelimited(KERN_INFO
     ^~~~~~~~~~~~~~~~~~

This was working before the rtc rework series because linux/ratelimit.h
was included implictly through asm/mach_traps.h -> asm/mc146818rtc.h
-> linux/mc146818rtc.h -> linux/rtc.h -> linux/device.h.

We clearly shouldn't rely on this indirect inclusion, so this adds
an explicit #include in the file that needs it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Fixes: 5ab788d7 ("rtc: cmos: move mc146818rtc code out of asm-generic/rtc.h")
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>