perf evsel: Actually show symbol offset in stack trace when requested

Symbol offset is one of the fields that can be requested in perf-script.
Currently you do not get that data when requested. e.g.,

perf script -f comm,tid,pid,time,cpu,sym,symoff,ip
...
gcc  6201/6201  [006] 762250.617897:
    ffffffff81090d95 update_curr
    ffffffff810911b8 dequeue_entity
    ffffffff81091825 dequeue_task_fair
    ffffffff81087163 dequeue_task
    ffffffff81087c03 deactivate_task
...

With this patch you get the offset:
...
gcc  6201/6201  [006] 762250.617897:
    ffffffff81090d95 update_curr+0x1c5
    ffffffff810911b8 dequeue_entity+0x28
    ffffffff81091825 dequeue_task_fair+0x45
    ffffffff81087163 dequeue_task+0x93
    ffffffff81087c03 deactivate_task+0x23
...

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1375024474-45726-1-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>