MIPS: Oprofile: Fix build as a module.

When building oprofile as a module for R10000 or R7000 class processors,
E9000 or MIPSxx class cores since 3572a2c3
[MIPS: make oprofile use cp0_perfcount_irq if it is set] an

ERROR: "cp0_compare_irq" [arch/mips/oprofile/oprofile.ko] undefined!

error will happen.  Fixed by exporting cp0_compare_irq.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>