Add missing RTC_EXPORT for the component build.

Bug: webrtc:9419
Change-Id: I3225259fb4cc55e9820f590928795f4587f1e3cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153884
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29479}
diff --git a/media/engine/webrtc_media_engine.h b/media/engine/webrtc_media_engine.h
index 40dbb90..0bd2ad5 100644
--- a/media/engine/webrtc_media_engine.h
+++ b/media/engine/webrtc_media_engine.h
@@ -27,6 +27,7 @@
 #include "media/base/media_engine.h"
 #include "modules/audio_device/include/audio_device.h"
 #include "modules/audio_processing/include/audio_processing.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace cricket {
 
@@ -52,7 +53,7 @@
 // CreateMediaEngine may be called on any thread, though the engine is
 // only expected to be used on one thread, internally called the "worker
 // thread". This is the thread Init must be called on.
-std::unique_ptr<MediaEngineInterface> CreateMediaEngine(
+RTC_EXPORT std::unique_ptr<MediaEngineInterface> CreateMediaEngine(
     MediaEngineDependencies dependencies);
 
 // Verify that extension IDs are within 1-byte extension range and are not