perf/core: Include perf_event_attr::defer_output in is_sb_event() check

Events configured with attr.defer_output = 1 receive deferred callchain
sideband records (PERF_RECORD_CALLCHAIN_DEFERRED) via perf_iterate_sb().

Include attr->defer_output in is_sb_event() so that such events are
attached to the sideband event list even when no other sideband flags
(such as mmap or comm) are enabled on the event.

Fixes: c69993ecdd4d ("perf: Support deferred user unwind")
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://patch.msgid.link/20261025215508.1237480-1-irogers@google.com