pmc: unhide struct pmc_md_dmc620_pmu_op_pmcallocate

Fix build after e3572eb654733a94e1e765fe9e95e0579981d851 as
struct pmc_md_dmc620_pmu_op_pmcallocate is needed when building
libpmc/pmclog.c as it is partof the public API via machine/pmc_mdep.h.