view_update_generator: Add logging to do_abort()

Just tell the logs that the guy is aborting
refs: #10941

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

db/view/view_update_generator.cc

@@ -233,6 +233,7 @@ void view_update_generator::do_abort() noexcept {
         return;
     }
 
+    vug_logger.info("Terminating background fiber");
     _db.unplug_view_update_generator();
     _as.request_abort();
     _pending_sstables.signal();