video_encoder: Add RTC_EXPORT ScalingSettings

ScalingSettings has some public constructors. These should be
able to be called from exteranl code. However, a linker fails
on windows because ScalingSettings doesn't have RTC_EXPORT.

Bug: chromium:1031965
Test: build crrev.com/c/1949841
Change-Id: Iddaea77f87c52edbe8f77551322d7aa198bc0aeb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161860
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30063}

api/video_codecs/video_encoder.h

diff --git a/api/video_codecs/video_encoder.h b/api/video_codecs/video_encoder.h
index 14dbf63..e5efda7 100644
--- a/api/video_codecs/video_encoder.h
+++ b/api/video_codecs/video_encoder.h
@@ -89,7 +89,7 @@
   };
 
   // Quality scaling is enabled if thresholds are provided.
-  struct ScalingSettings {
+  struct RTC_EXPORT ScalingSettings {
    private:
     // Private magic type for kOff, implicitly convertible to
     // ScalingSettings.