perf python scripting: print the syscall name on sctop

[root@emilia tmp]# perf trace sctop 1
syscall events:

event                                          count
----------------------------------------  ----------
read                                          215400
futex                                           4029
write                                            376
brk                                               33
rt_sigprocmask                                    24
select                                            17
lseek                                              2
fsync                                              1
^C[root@emilia tmp]#

Cc: David S. Miller <davem@davemloft.net>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>