perf top: Suppress DSO column if only one is present

perf top: Suppress DSO column if only one is present

E.g.

[root@doppio ~]# perf top -U
---------------------------------------------------------------------------
   PerfTop:     482 irqs/sec  kernel:100.0% [1000Hz cycles],  (all, 2 CPUs)
---------------------------------------------------------------------------
DSO: vmlinux
             samples  pcnt function
             _______ _____ _________________________

              471.00 47.9% read_hpet
               57.00  5.8% acpi_os_read_port
               30.00  3.1% hpet_next_event
               30.00  3.1% find_busiest_group
               22.00  2.2% schedule
               18.00  1.8% sched_clock_local
               14.00  1.4% _spin_lock_irqsave
               14.00  1.4% native_read_tsc
               13.00  1.3% trace_hardirqs_off
                9.00  0.9% fget_light
                9.00  0.9% ioread8
                8.00  0.8% do_sys_poll

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1258479655-28662-3-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>