[MIPS] I8253: Export i2853_lock to modules.

This fixes:

  ERROR: "i8253_lock" [drivers/input/misc/pcspkr.ko] undefined!
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/kernel/i8253.c

@@ -15,6 +15,7 @@
 #include <asm/time.h>
 
 DEFINE_SPINLOCK(i8253_lock);
+EXPORT_SYMBOL(i8253_lock);
 
 /*
  * Initialize the PIT timer.