MIPS: perf: Fix build error caused by unused counters_per_cpu_to_total()

cc1: warnings being treated as errors
arch/mips/kernel/perf_event_mipsxx.c:166: error: 'counters_per_cpu_to_total' defined but not used
make[2]: *** [arch/mips/kernel/perf_event_mipsxx.o] Error 1
make[2]: *** Waiting for unfinished jobs....

It was first introduced by 82091564

 [MIPS:
perf: Add support for 64-bit perf counters.] in 3.2.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: david.daney@cavium.com
Patchwork: https://patchwork.linux-mips.org/patch/3357/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>