Add missing RTC_EXPORT for the Chromium Windows build.

After fixing the issue with crbug.com/1018579, lld-link complained
that some symbols need to be exported, see [1].

[1] - https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/432025

Bug: webrtc:9419
Change-Id: I9107a9c76361f4c66463a9af2e81a3991ae14df5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159007
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29721}

api/video_codecs/video_encoder.h

@@ -144,7 +144,7 @@ class RTC_EXPORT VideoEncoder {
   };
 
   // Struct containing metadata about the encoder implementing this interface.
-  struct EncoderInfo {
+  struct RTC_EXPORT EncoderInfo {
     static constexpr uint8_t kMaxFramerateFraction =
         std::numeric_limits<uint8_t>::max();