Add libplatform deps file

Add deps file allowing libplatform.h to include v8-tracing.h.

Additionally removes redundant include/ that was causing build errors
for node-lkgr.

BUG=

Review-Url: https://codereview.chromium.org/2195403002
Cr-Commit-Position: refs/heads/master@{#38217}

include/libplatform/DEPS

+specific_include_rules = {
+  "libplatform\.h": [
+    "+libplatform/v8-tracing.h",
+  ],
+}

include/libplatform/libplatform.h

@@ -5,7 +5,7 @@
 #ifndef V8_LIBPLATFORM_LIBPLATFORM_H_
 #define V8_LIBPLATFORM_LIBPLATFORM_H_
 
-#include "include/libplatform/v8-tracing.h"
+#include "libplatform/v8-tracing.h"
 #include "v8-platform.h"  // NOLINT(build/include)
 
 namespace v8 {