build: add missing src/tracing header files

I noticed that only one header from src/tracing is included in the
sources list in node.gyp. Not sure if this is intentional or not so I
wanted to bring it up just in case this was overlooked.

PR-URL: nodejs#10851
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

node.gyp

@@ -234,6 +234,9 @@
         'src/stream_base.h',
         'src/stream_base-inl.h',
         'src/stream_wrap.h',
+        'src/tracing/agent.h',
+        'src/tracing/node_trace_buffer.h',
+        'src/tracing/node_trace_writer.h',
         'src/tracing/trace_event.h'
         'src/tree.h',
         'src/util.h',