Export symbols needed by the Chromium component build (part 8).

This CL uses RTC_EXPORT (defined in rtc_base/system/rtc_export.h)
to mark WebRTC symbols as visible from a shared library, this doesn't
mean these symbols are part of the public API (please continue to refer
to [1] for info about what is considered public WebRTC API).

[1] - https://webrtc.googlesource.com/src/+/HEAD/native-api.md

Bug: webrtc:9419
Change-Id: Ib2c29054b2ae008f5291bd3b762a504b18534326
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/130513
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27410}

api/video_codecs/video_encoder_software_fallback_wrapper.h

@@ -14,13 +14,15 @@
 #include <memory>
 
 #include "api/video_codecs/video_encoder.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
 // Used to wrap external VideoEncoders to provide a fallback option on
 // software encoding when a hardware encoder fails to encode a stream due to
 // hardware restrictions, such as max resolution.
-std::unique_ptr<VideoEncoder> CreateVideoEncoderSoftwareFallbackWrapper(
+RTC_EXPORT std::unique_ptr<VideoEncoder>
+CreateVideoEncoderSoftwareFallbackWrapper(
     std::unique_ptr<VideoEncoder> sw_fallback_encoder,
     std::unique_ptr<VideoEncoder> hw_encoder);