Fix the iOS Framework dependency

`Foundation.framework` is not just for mac build, it is also needed on iOS build.

Bug: None
Change-Id: I94694102afbebbe60182521892e51c57760eb7c2
Reviewed-on: https://webrtc-review.googlesource.com/47656
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Jiawei Ou <ouj@fb.com>
Cr-Commit-Position: refs/heads/master@{#21919}

rtc_base/BUILD.gn

@@ -39,6 +39,7 @@ config("rtc_base_all_dependent_config") {
   if (is_ios) {
     libs = [
       "CFNetwork.framework",
+      "Foundation.framework",
       "Security.framework",
       "SystemConfiguration.framework",
       "UIKit.framework",