perf script: Fixup some evsel/evlist method names

Fixups related to the introduction of libperf, where the
perf_{evsel,evlist}__ prefix is reserved for functions operating on
struct perf_{evsel,evlist}.

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