Mark trace-only variable as unused to fix build errors

Tracing can be disabled by setting the build flag
rtc_disable_trace_events = true

This causes the variable to be unused.

Bug: webrtc:12787
Change-Id: Iebbb8cbb5ede5453ad24ce7710de3b1dd68ad83f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/261683
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Markus Handell <handellm@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36843}

pc/proxy.h

@@ -94,7 +94,7 @@ class ScopedTrace {
   ~ScopedTrace();
 
  private:
-  const char* const class_and_method_name_;
+  [[maybe_unused]] const char* const class_and_method_name_;
 };
 }  // namespace proxy_internal