perf symbols: Consider unresolved DSOs in the dso__col_widt calculation

perf symbols: Consider unresolved DSOs in the dso__col_widt calculation

By using BITS_PER_LONG / 4, that is the number of chars that will be
used in such cases as the DSO "name".

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>