src: move node_trace_writer/buffer.h to agent.cc

The headers node_trace_writer.h and node_trace_buffer.h are not used in
agent.h but are more of an implementation detail of agent.cc.

This commit suggests moving the inclusion of these headers to agent.cc.

PR-URL: https://github.com/nodejs/node/pull/15598


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>