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There we need just some struct forward declarations, do that instead and
add the includes needed by metricgroup.c.

That should help with needless rebuilds when changing the removed
headers from metricgroup.h.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-1fkskjws6imir2hhztqhdyb0@git.kernel.org

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>