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}

rtc_base/rtc_certificate.h

@@ -18,6 +18,7 @@
 
 #include "api/scoped_refptr.h"
 #include "rtc_base/ref_count.h"
+#include "rtc_base/system/rtc_export.h"
 
 namespace rtc {
 
@@ -48,7 +49,7 @@ class RTCCertificatePEM {
 // A thin abstraction layer between "lower level crypto stuff" like
 // SSLCertificate and WebRTC usage. Takes ownership of some lower level objects,
 // reference counting protects these from premature destruction.
-class RTCCertificate : public RefCountInterface {
+class RTC_EXPORT RTCCertificate : public RefCountInterface {
  public:
   // Takes ownership of |identity|.
   static scoped_refptr<RTCCertificate> Create(