Move sigslot to proper third_party directory

Extract sigslot into separate target and move it to proper third_party
directory.

Bug: webrtc:8366
Change-Id: Id2e0712bd020bfad811947803c94553dce06d976
Reviewed-on: https://webrtc-review.googlesource.com/84141
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24099}
diff --git a/pc/datachannel.h b/pc/datachannel.h
index 4491657..d29ee59 100644
--- a/pc/datachannel.h
+++ b/pc/datachannel.h
@@ -21,7 +21,7 @@
 #include "pc/channel.h"
 #include "rtc_base/messagehandler.h"
 #include "rtc_base/scoped_ref_ptr.h"
-#include "rtc_base/sigslot.h"
+#include "rtc_base/third_party/sigslot/sigslot.h"
 
 namespace webrtc {