MIPS: Export get_c0_perfcount_int()

MIPS: Export get_c0_perfcount_int()

get_c0_perfcount_int is tested from oprofile code. If oprofile is
compiled as module, get_c0_perfcount_int needs to be exported, otherwise
it cannot be resolved.

Fixes: a669efc4a3b4 ("MIPS: Add hook to get C0 performance counter interrupt")
Cc: stable@vger.kernel.org # v3.19+
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: abrestic@chromium.org
Patchwork: https://patchwork.linux-mips.org/patch/10763/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>