[NaCl Add] Define WIN32 by default for PPAPI host builds

BUG=

Review URL: https://codereview.chromium.org/11421016

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1469 050acbb0-2703-11df-ab0a-9f3f633ae91d
1 file changed