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/api/audio_codecs/isac/BUILD.gn b/api/audio_codecs/isac/BUILD.gn
index 719aee6..469176f 100644
--- a/api/audio_codecs/isac/BUILD.gn
+++ b/api/audio_codecs/isac/BUILD.gn
@@ -79,6 +79,7 @@
     "..:audio_codecs_api",
     "../../../modules/audio_coding:isac_fix",
     "../../../rtc_base:rtc_base_approved",
+    "../../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/strings",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
@@ -95,6 +96,7 @@
     "..:audio_codecs_api",
     "../../../modules/audio_coding:isac_fix",
     "../../../rtc_base:rtc_base_approved",
+    "../../../rtc_base/system:rtc_export",
     "//third_party/abseil-cpp/absl/strings",
     "//third_party/abseil-cpp/absl/types:optional",
   ]