scons: Set the default windows platform to be windows userspace.

I thought I had done this ages ago.
diff --git a/common.py b/common.py
index 3b6bf52..101fc55 100644
--- a/common.py
+++ b/common.py
@@ -12,7 +12,7 @@
 
 _platform_map = {
 	'linux2': 'linux',
-	'win32': 'winddk',
+	'win32': 'windows',
 }
 
 default_platform = sys.platform