perf symbols: Call the symbol filter in dso__synthesize_plt_symbols()

We need to pass the symbol to the filter so that, for instance,
'perf top' can do filtering and also set the private area it
manages, setting the ->map pointer, etc.

I found this while running 'perf top' on a machine where hits
happened on PLT symbols, where ->map wasn't being set up and
segfaults thus happened.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1258386491-20278-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>