Mark SimulcastStream RTC_EXPORT

Bug: b:320555128
Test: Build
Change-Id: Ia0a8b7a3e49c8791fe240b2ab38e4779c2af9f24
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/351440
Commit-Queue: Hirokazu Honda <hiroh@google.com>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42434}

api/video_codecs/simulcast_stream.h

@@ -18,7 +18,7 @@ namespace webrtc {
 
 // TODO(bugs.webrtc.org/6883): Unify with struct VideoStream, part of
 // VideoEncoderConfig.
-struct SimulcastStream {
+struct RTC_EXPORT SimulcastStream {
   // Temporary utility methods for transition from numberOfTemporalLayers
   // setting to ScalabilityMode.
   unsigned char GetNumberOfTemporalLayers() const;