Allow building without sctp on mac/ios.

Bug: webrtc:8590
Change-Id: I4388a2292d3d5dcdb5fdcc11a1807e4790261565
Reviewed-on: https://webrtc-review.googlesource.com/27985
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21077}

media/BUILD.gn

@@ -265,6 +265,11 @@ rtc_static_library("rtc_data") {
       "sctp/sctptransport.h",
       "sctp/sctptransportinternal.h",
     ]
+  } else {
+    # libtool on mac does not like empty targets.
+    sources = [
+      "//rtc_base/noop.cc",
+    ]
   }
 
   configs += [ ":rtc_media_warnings_config" ]