constants: add O_DIRECT

This will allow to speed up file i/o in some cases by usage of right offsets
and buffer sizes.

src/node_constants.cc

@@ -106,6 +106,9 @@ void DefineConstants(Handle<Object> target) {
   NODE_DEFINE_CONSTANT(target, O_SYMLINK);
 #endif
 
+#ifdef O_DIRECT
+  NODE_DEFINE_CONSTANT(target, O_DIRECT);
+#endif
 
 #ifdef S_IRWXU
   NODE_DEFINE_CONSTANT(target, S_IRWXU);