libperf: Move 'pollfd' from 'struct evlist' to 'struct perf_evlist'

libperf: Move 'pollfd' from 'struct evlist' to 'struct perf_evlist'

Moving 'pollfd' from 'struct evlist' to 'struct perf_evlist' it will be
used in following patches.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lore.kernel.org/lkml/20190913132355.21634-23-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>