build: move source files from headers section

Currently, the sources list contains sources and headers which are
separated by a comment. I noticed two .cc files after the headers
comment and this commit moves those files the start of the list
where the rest of source files are.

PR-URL: https://github.com/nodejs/node/pull/10850
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

@@ -171,6 +171,7 @@
         'src/signal_wrap.cc',
         'src/spawn_sync.cc',
         'src/string_bytes.cc',
+        'src/string_search.cc',
         'src/stream_base.cc',
         'src/stream_wrap.cc',
         'src/tcp_wrap.cc',
@@ -178,6 +179,7 @@
         'src/tty_wrap.cc',
         'src/process_wrap.cc',
         'src/udp_wrap.cc',
+        'src/util.cc',
         'src/uv.cc',
         # headers to make for a more pleasant IDE experience
         'src/async-wrap.h',
@@ -218,8 +220,6 @@
         'src/tree.h',
         'src/util.h',
         'src/util-inl.h',
-        'src/util.cc',
-        'src/string_search.cc',
         'deps/http_parser/http_parser.h',
         'deps/v8/include/v8.h',
         'deps/v8/include/v8-debug.h',