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: #10851
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Italo A. Casas <[email protected]>

Author: danbev

node.gyp

@@ -226,6 +226,9 @@
         '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',