src: add include guards to internal headers

For consistency with the newly added src/base64.h header, check that
NODE_WANT_INTERNALS is defined and set in internal headers.

PR-URL: https://github.com/nodejs/node/pull/6948
Refs: https://github.com/nodejs/node/pull/6910
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

src/string_bytes.h

@@ -1,6 +1,8 @@
 #ifndef SRC_STRING_BYTES_H_
 #define SRC_STRING_BYTES_H_
 
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
 // Decodes a v8::Local<v8::String> or Buffer to a raw char*
 
 #include "v8.h"
@@ -108,4 +110,6 @@ class StringBytes {
 
 }  // namespace node
 
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
 #endif  // SRC_STRING_BYTES_H_