build: add missing comma in sources list

This commit adds a missing comma in the sources list. This effects at
least window which produces the following warning where trace_event.h
and src/util.h are concatenated:

Warning: Missing input files:
src\tracing\trace_event.hsrc\util.h

node.gyp

@@ -284,7 +284,7 @@
         'src/tracing/agent.h',
         'src/tracing/node_trace_buffer.h',
         'src/tracing/node_trace_writer.h',
-        'src/tracing/trace_event.h'
+        'src/tracing/trace_event.h',
         'src/util.h',
         'src/util-inl.h',
         'deps/http_parser/http_parser.h',