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/api/rtc_error.h b/api/rtc_error.h
index ffdcc0a..970507f 100644
--- a/api/rtc_error.h
+++ b/api/rtc_error.h
@@ -131,7 +131,7 @@
 //
 // Only intended to be used for logging/diagnostics. The returned char* points
 // to literal string that lives for the whole duration of the program.
-const char* ToString(RTCErrorType error);
+RTC_EXPORT const char* ToString(RTCErrorType error);
 
 #ifdef UNIT_TEST
 inline std::ostream& operator<<(  // no-presubmit-check TODO(webrtc:8982)