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/rtc_base/sigslottester_unittest.cc b/rtc_base/sigslottester_unittest.cc
index c8e87e5..8fde268 100644
--- a/rtc_base/sigslottester_unittest.cc
+++ b/rtc_base/sigslottester_unittest.cc
@@ -13,7 +13,7 @@
 #include <string>
 
 #include "rtc_base/gunit.h"
-#include "rtc_base/sigslot.h"
+#include "rtc_base/third_party/sigslot/sigslot.h"
 
 namespace rtc {