Add missing BASE_EXPORT to base::AsyncDispatcher declaration.

This fixes the component build for Fuchsia.

TBR: sergeyu
Bug: 831384
Change-Id: Ifd335abe1f1013e28b0c266b525312b704b021e6
Reviewed-on: https://chromium-review.googlesource.com/1012587Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550697}

base/fuchsia/async_dispatcher.h

@@ -18,7 +18,7 @@ namespace base {
 
 // Implementation of dispatcher for Fuchsia's async library. It's necessary to
 // run Fuchsia's library on chromium threads.
-class AsyncDispatcher : public async_t {
+class BASE_EXPORT AsyncDispatcher : public async_t {
  public:
   AsyncDispatcher();
   ~AsyncDispatcher();
@@ -81,4 +81,4 @@ class AsyncDispatcher : public async_t {
 
 }  // namespace base
 
-#endif  // BASE_FUCHSIA_ASYNC_DISPATCHER_H_
\ No newline at end of file
+#endif  // BASE_FUCHSIA_ASYNC_DISPATCHER_H_