perf metricgroup: Remove needless includes from metricgroup.h

perf metricgroup: Remove needless includes from metricgroup.h

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>