Mark AsyncInvoker as deprecated

Also fix similar annotation on NackModule to have effect
(when defining an alias with C++ using, ABSL_DEPRECATED should appear
on the left hand side).

Bug: webrtc:12339
Change-Id: Id80a20bf2c56a826777b8a40e06ac5c65e4f8db7
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217242
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33905}
diff --git a/modules/video_coding/deprecated/nack_module.h b/modules/video_coding/deprecated/nack_module.h
index 0ad009b..2fac6ce 100644
--- a/modules/video_coding/deprecated/nack_module.h
+++ b/modules/video_coding/deprecated/nack_module.h
@@ -125,7 +125,7 @@
   const absl::optional<BackoffSettings> backoff_settings_;
 };
 
-using NackModule = ABSL_DEPRECATED("") DEPRECATED_NackModule;
+using NackModule ABSL_DEPRECATED("") = DEPRECATED_NackModule;
 
 }  // namespace webrtc