perf hists browser: React to unassigned hotkey pressing

When that happens we were just ignoring the key press, now this
message is presented in the bottom line (the help line):

  "Press '?' for help on key bindings"

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-iyma2j5kj3q9i1stl4mfh90n@git.kernel.org


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