Fix #include order in snapshot-external.cc

This corrects a linter complaint.

BUG=
R=mstarzinger@chromium.org, vogelheim@chromium.org

Review URL: https://codereview.chromium.org/351643002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21947 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/snapshot-external.cc

@@ -6,9 +6,9 @@
 
 #include "src/snapshot.h"
 
-#include "src/v8.h"  // for V8::Initialize
 #include "src/serialize.h"
 #include "src/snapshot-source-sink.h"
+#include "src/v8.h"  // for V8::Initialize
 
 namespace v8 {
 namespace internal {