perf symbols: Initialize dso->loaded

Brown paper bag bug introduced in:

 66bd8424 ("perf tools: Delay
 loading symtabs till we hit a map with it")

Without this we were not loading any symtabs that happened to be
on a DSO for which the allocated memory for ->loaded was !0.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1257270738-5669-1-git-send-email-acme@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>