perf ui: Make 'exit_msg' optional in ui__question_window()

We will not need it when refactoring this function to be
non-interactive, so make it optional.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-pnx1dn17bsz7lqt9ty95nnjx@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>