perf tools: move perf_pmus__find_core_pmu() prototype to pmus.h

perf_pmus__find_core_pmu() is implemented in util/pmus.c but its
prototpye is in util/pmu.h. Move it to util/pmus.h.

Suggested-by: Ian Rogers <irogers@google.com>
Signed-off-by: Thomas Falcon <thomas.falcon@intel.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250612163659.1357950-1-thomas.falcon@intel.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>