Make MSVS happy with ExternalAsciiStringResourceImpl

TEST=MSVS 2008 no longer generates warning C4251 for ExternalAsciiStringResourceImpl

Review URL: http://codereview.chromium.org/8056019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9462 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -2452,7 +2452,7 @@ class V8EXPORT TypeSwitch : public Data {
 
 // --- Extensions ---
 
-class ExternalAsciiStringResourceImpl
+class V8EXPORT ExternalAsciiStringResourceImpl
     : public String::ExternalAsciiStringResource {
  public:
   ExternalAsciiStringResourceImpl() : data_(0), length_(0) {}