Export symbols needed by the Chromium component build (part 1).

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: I802abd32874d42d3aa5ecd3c8022e7cf5e043d99
Reviewed-on: https://webrtc-review.googlesource.com/c/103505
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24969}

modules/audio_processing/include/config.h

@@ -14,6 +14,7 @@
 #include <map>
 
 #include "rtc_base/constructormagic.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -57,7 +58,7 @@ enum class ConfigOptionID {
 //    config.Set<Algo1_CostFunction>(new SqrCost());
 //
 // Note: This class is thread-compatible (like STL containers).
-class Config {
+class RTC_EXPORT Config {
  public:
   // Returns the option if set or a default constructed one.
   // Callers that access options too often are encouraged to cache the result.