Add Foundation.framework to cocoa_threading target

https://webrtc-review.googlesource.com/c/src/+/105301 remove the dependency to rtc_base_generic, it also removed the dependnecy to Foundation.framework. This CL adds it back.

Bug: webrtc:9838
Change-Id: I861e73d13eb36d2c3a09d998a6def9512066f0d5
Reviewed-on: https://webrtc-review.googlesource.com/c/122621
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jiawei Ou <ouj@fb.com>
Cr-Commit-Position: refs/heads/master@{#26654}

rtc_base/system/BUILD.gn

@@ -75,5 +75,6 @@ if (is_mac || is_ios) {
     deps = [
       "..:checks",
     ]
+    libs = [ "Foundation.framework" ]
   }
 }