perf evsel: Move xyarray.h from evsel.c to evsel.h to reduce include dep tree

All we need in util/evsel.h is the foward declaration of 'struct
xyarray', not the internal/xyarray.h, that can be moved to util/evsel.c
and then we reduce the header dependency tree.

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-wwqce6ixwcyq6yzx3ljrdm80@git.kernel.org


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