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/api/crypto/BUILD.gn b/api/crypto/BUILD.gn
index a9bd479..685dccd 100644
--- a/api/crypto/BUILD.gn
+++ b/api/crypto/BUILD.gn
@@ -24,6 +24,7 @@
   ]
   deps = [
     "../../rtc_base:rtc_base",
+    "../../rtc_base/system:rtc_export",
   ]
 }