Deprecate the StreamInterface::SignalEvent sigslot

In its stead, there's now a SetEventCallback() method.

Bug: webrtc:11943
Change-Id: If936d6e1e23e8a584f06feb123ecf2d450ea4145
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/319040
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42187}
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index a7a2ada..f761c36 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1464,6 +1464,8 @@
     "system:rtc_export",
     "third_party/sigslot",
   ]
+
+  absl_deps = [ "//third_party/abseil-cpp/absl/functional:any_invocable" ]
 }
 
 rtc_library("rtc_certificate_generator") {
@@ -2131,6 +2133,7 @@
           "ssl_identity_unittest.cc",
           "ssl_stream_adapter_unittest.cc",
         ]
+        deps += [ ":callback_list" ]
       }
       absl_deps = [
         "//third_party/abseil-cpp/absl/algorithm:container",