perf top: Add a visual cue for toggle zeroing of samples

When 'perf top' is run, one can't easily find a difference
between -z option and normal output.
So I added a visual cue to know whether it is the zeroing or not.

Output is as below.

Before:
 $ perf top

 Samples: 61K of event 'cycles', Event count (approx.): 3908136933
 Overhead  Shared Object                       Symbol
    1.42%  firefox                             [.] 0x0000000000011e76
    1.32%  libpthread-2.17.so                  [.] pthread_mutex_lock

if you press key 'z' or run with zero option like '$ perf top --zero', it is as below.

After:
 Samples: 61K of event 'cycles', Event count (approx.): 3908136933 [z]
 Overhead  Shared Object                       Symbol
    1.42%  firefox                             [.] 0x0000000000011e76
    1.32%  libpthread-2.17.so                  [.] pthread_mutex_lock

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>