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}

rtc_base/BUILD.gn

@@ -186,6 +186,7 @@ rtc_library("criticalsection") {
     ":checks",
     ":macromagic",
     ":platform_thread_types",
+    "system:rtc_export",
     "system:unused",
   ]
 }