rgw: fix initialization of s3_event in pubsub

rgw: fix initialization of s3_event in pubsub

resolves compiler warning:

rgw_sync_module_pubsub.cc: In constructor ‘RGWPSHandleObjEventCR::RGWPSHandleObjEventCR(RGWDataSyncCtx*, PSEnvRef, const rgw_user&, EventRef<rgw_pubsub_event>&, EventRef<rgw_pubsub_s3_event>&, const TopicsRef&)’:
rgw_sync_module_pubsub.cc:1028:3: warning: ‘RGWPSHandleObjEventCR::s3_event’ is initialized with itself [-Winit-self]
 1028 |   RGWPSHandleObjEventCR(RGWDataSyncCtx* const _sc,
      |   ^~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Casey Bodley <cbodley@redhat.com>