ARM: perf: fix prototype of release_pmu

Commit  f12482c9 ("ARM: 6974/1: pmu: refactor reservation") changed the
prototype of release_pmu, but missed the stub for when
CONFIG_CPU_HAS_PMU is not selected by the platform.

This patch changes the prototype of the stub, preventing possible build
failures when CONFIG_CPU_HAS_PMU is not selected.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>