perf trace: Only insert blank duration bracket when tracing syscalls

perf trace: Only insert blank duration bracket when tracing syscalls

When printing just events, i.e. '--no-sys --ev some:events' it makes no
sense to waste screen space.

Before:

 # trace --no-sys --ev probe:*
 84481.704 (         ): probe:vfs_getname:(ffffffff811ed023) pathname="/etc/services")
 84481.892 (         ): probe:vfs_getname:(ffffffff811ed023) pathname="/etc/services")
 84482.230 (         ): probe:vfs_getname:(ffffffff811ed023) pathname="/etc/resolv.conf")
 84482.481 (         ): probe:vfs_getname:(ffffffff811ed023) pathname="/etc/hosts")
 85097.725 (         ): probe:vfs_getname:(ffffffff811ed023) pathname="/root"
 #

After:

 # trace --no-sys --ev probe:*
 0.000 probe:vfs_getname:(ffffffff811ed023) pathname="/root")
 1.711 probe:vfs_getname:(ffffffff811ed023) pathname="/etc/localtime")
 2.103 probe:vfs_getname:(ffffffff811ed023) pathname="/etc/localtime")
^C#

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: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-jhryxgnam8zecq0q0wsy6pyb@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>