perfctr: cleanup a warning
perfctr: cleanup a warning

Include papi_libpfm_events.h for _papi_libpfm_init() decl.

pyrovski/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,arm-hpc/papi,arm-hpc/papi,pyrovski/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi