RTC_EXPORT AsyncDnsResolver

Its predecessor is being used directly by at least one Chromium function.

Bug: webrtc:12598
Change-Id: I0c521f03cf6664036a48d5d45dcacaa74ae8582c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/322800
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40896}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index fdc2adf..419128d 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -940,6 +940,7 @@
     "../api:make_ref_counted",
     "../api:sequence_checker",
     "../api/task_queue:pending_task_safety_flag",
+    "system:rtc_export",
   ]
 }