perf tests: Set evlist of test__sw_clock_freq() to !overwrite

perf tests: Set evlist of test__sw_clock_freq() to !overwrite

Unsetting overwrite when calling perf_evlist__mmap is harmless. This
commit passes false to it, makes following commits eliminate the
overwrite argument easier.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20171113013809.212417-4-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>