perf hists browser: Do not exit on tab key with single event

perf hists browser: Do not exit on tab key with single event

TUI help states for multiple event sessions the TAB/UNTAB keys are used
to switch events. For single event sessions (e.g., the default) the tab
key currently causes the tui to exit. Change that to do nothing since
there is not no second event to switch to.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1319045867-12728-1-git-send-email-dsahern@gmail.com
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>