Attempt to fix the Windows bot

Unreviewed.

Patch by Benjamin Poulain <bpoulain@apple.com> on 2013-04-05

* JavaScriptCore.vcproj/JavaScriptCore/JavaScriptCoreExports.def:
* JavaScriptCore.vcxproj/JavaScriptCoreExportGenerator/JavaScriptCoreExports.def.in:
r147825 removed the symbol for nullptr_t. Add it back.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147827 268f45cc-cd09-0410-ab3c-d52691b4dbfc