perfetto: build with '-std=c++20'

abseil-cpp is built with C++20, perfetto must use the same version.

Fixes: https://github.com/NixOS/nixpkgs/issues/568538

pkgs/by-name/pe/perfetto/package.nix

@@ -80,6 +80,7 @@ let
   toGnFlags = lib.mapAttrsToList (name: value: "${name}=${toGnValue value}");
 
   commonGnFlags = {
+    extra_cxxflags = "-std=c++20";
     is_debug = false;
     is_system_compiler = true;
     is_clang = stdenv.cc.isClang;