perf python: Update ungrouped evsel leader in clone

perf python: Update ungrouped evsel leader in clone

evsels are cloned in the python code as they form part of the Python
object pyrf_evsel. The cloning doesn't update the evsel's leader, do
this for the case of an evsel being ungrouped.

Reviewed-by: Howard Chu <howardchu95@gmail.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20250228222308.626803-8-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>