Mark class as exported to fix win build.

TBR=machenbach@chromium.org
BUG=chromium:470930
LOG=N

Review URL: https://codereview.chromium.org/1149923006

Cr-Commit-Position: refs/heads/master@{#28647}

include/v8.h

@@ -1198,7 +1198,7 @@ class V8_EXPORT ScriptCompiler {
    * For streaming incomplete script data to V8. The embedder should implement a
    * subclass of this class.
    */
-  class ExternalSourceStream {
+  class V8_EXPORT ExternalSourceStream {
    public:
     virtual ~ExternalSourceStream() {}