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/rtc_base/async_resolver_interface.h b/rtc_base/async_resolver_interface.h
index 0f5e989..0d0bb7c 100644
--- a/rtc_base/async_resolver_interface.h
+++ b/rtc_base/async_resolver_interface.h
@@ -12,12 +12,13 @@
 #define RTC_BASE_ASYNC_RESOLVER_INTERFACE_H_
 
 #include "rtc_base/socket_address.h"
+#include "rtc_base/system/rtc_export.h"
 #include "rtc_base/third_party/sigslot/sigslot.h"
 
 namespace rtc {
 
 // This interface defines the methods to resolve the address asynchronously.
-class AsyncResolverInterface {
+class RTC_EXPORT AsyncResolverInterface {
  public:
   AsyncResolverInterface();
   virtual ~AsyncResolverInterface();