ARM: PMU: re-export release_pmu symbol to modules

Commit b0e89590 ("ARM: PMU: move CPU PMU platform device handling and
init into perf") inadvertently removed the EXPORT_SYMBOL_GPL on
release_pmu, so out-of-tree modules can no longer play nice with perf,
even if they tried in the first place.

This patch re-exports the symbol.
Reported-by: NJon Medhurst (Tixy) <jon.medhurst@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm/kernel/pmu.c

@@ -33,3 +33,4 @@ release_pmu(enum arm_pmu_type type)
 {
 	clear_bit_unlock(type, pmu_lock);
 }
+EXPORT_SYMBOL_GPL(release_pmu);