perf evsel: Convert last 'struct evsel' methods to the right evsel__ prefix

As 'perf_evsel__' means its a function in tools/lib/perf/.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>