Adding missing RTC_EXPORT for component build.

Bug: webrtc:9419
Change-Id: Ifa5d21edc708b5012b71e2e5101e10c6352a7218
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157162
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29533}
diff --git a/rtc_base/critical_section.h b/rtc_base/critical_section.h
index a13721e..cf10463 100644
--- a/rtc_base/critical_section.h
+++ b/rtc_base/critical_section.h
@@ -14,6 +14,7 @@
 #include "rtc_base/checks.h"
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/platform_thread_types.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/thread_annotations.h"
 
 #if defined(WEBRTC_WIN)
@@ -45,7 +46,7 @@
 // Locking methods (Enter, TryEnter, Leave)are const to permit protecting
 // members inside a const context without requiring mutable CriticalSections
 // everywhere. CriticalSection is reentrant lock.
-class RTC_LOCKABLE CriticalSection {
+class RTC_LOCKABLE RTC_EXPORT CriticalSection {
  public:
   CriticalSection();
   ~CriticalSection();