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/BUILD.gn b/BUILD.gn
index f29fbdd..d337738 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -444,6 +444,7 @@
       "rtc_base:rtc_numerics_unittests",
       "rtc_base:rtc_task_queue_unittests",
       "rtc_base:sequenced_task_checker_unittests",
+      "rtc_base:sigslot_unittest",
       "rtc_base:weak_ptr_unittests",
       "rtc_base/experiments:experiments_unittests",
       "system_wrappers:metrics_default",