win,build: remove duplicate definition

The duplicate warning was fixed by e1fe270, but returned as of 9f45799.
Hopefully this will fix the issue for good.

PR: nodejs#512
Reviewed-by: Ben Noordhuis <[email protected]>

Author: piscisaureus

node.gyp

@@ -322,6 +322,9 @@
           'sources': [
             'src/res/node.rc',
           ],
+          'defines!': [
+            'NODE_PLATFORM="win"',
+          ],
           'defines': [
             'FD_SETSIZE=1024',
             # we need to use node's preferred "win32" rather than gyp's preferred "win"