perf evlist: Fix typo in perf_evlist__start_workload()

perf evlist: Fix typo in perf_evlist__start_workload()

This patch fixes a typo: s/enable to/unable to/

Signed-off-by: Soramichi AKIYAMA <akiyama@m.soramichi.jp>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: bcf3145fbeb1 ("perf evlist: Enhance perf_evlist__start_workload()")
Link: http://lkml.kernel.org/r/20170110200006.e1f7a766b4faf1f107ae2e1b@m.soramichi.jp
[ Wasn't applying, fixed it up by hand, added Fixes: tag ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>