Add the perf_evlist and perf_evsel structs to libperf.

It's added as a declarations into:

  include/perf/evlist.h
  include/perf/evsel.h

which will be included by users.

The definitions are added into:

  include/internal/evlist.h
  include/internal/evsel.h

which is not to be included by users, but shared
within perf and libperf.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190721112506.12306-35-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>