perf kvm stat: Replace kvm-stat.h includes with forward declarations

To reduce the include header dependency tree and speed up perf builds.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-dngwaxuhfnhksawgdpo6e74n@git.kernel.org


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