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/p2p/BUILD.gn b/p2p/BUILD.gn
index e62822c..c319fc2 100644
--- a/p2p/BUILD.gn
+++ b/p2p/BUILD.gn
@@ -90,6 +90,7 @@
     "../rtc_base:safe_minmax",
     "../rtc_base:stringutils",
     "../rtc_base/third_party/base64",
+    "../rtc_base/third_party/sigslot",
     "../system_wrappers:field_trial_api",
     "../system_wrappers:metrics_api",
     "//third_party/abseil-cpp/absl/memory",
@@ -139,6 +140,7 @@
       "../rtc_base:rtc_base",
       "../rtc_base:rtc_base_approved",
       "../rtc_base:rtc_base_tests_utils",
+      "../rtc_base/third_party/sigslot",
       "../test:test_support",
       "//third_party/abseil-cpp/absl/memory",
     ]