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/dtls_transport_interface.h b/api/dtls_transport_interface.h
index 1170e0f..8fd8cd1 100644
--- a/api/dtls_transport_interface.h
+++ b/api/dtls_transport_interface.h
@@ -20,6 +20,7 @@
 #include "api/scoped_refptr.h"
 #include "rtc_base/ref_count.h"
 #include "rtc_base/ssl_certificate.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace webrtc {
 
@@ -37,7 +38,7 @@
 
 // This object gives snapshot information about the changeable state of a
 // DTLSTransport.
-class DtlsTransportInformation {
+class RTC_EXPORT DtlsTransportInformation {
  public:
   DtlsTransportInformation();
   explicit DtlsTransportInformation(DtlsTransportState state);