Fixed shared_library build on Windows.

BUG=v8:2879
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/24054008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16729 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -819,7 +819,7 @@ class V8_EXPORT HandleScope {
  * value.
  */
 template<class T>
-struct Maybe {
+struct V8_EXPORT Maybe {
   Maybe() : has_value(false) {}
   explicit Maybe(T t) : has_value(true), value(t) {}
   Maybe(bool has, T t) : has_value(has), value(t) {}