Fixes for building with w32api headers on Mingw.

diff --git a/dlls/twain/capability.c b/dlls/twain/capability.c
index 28311e7..fb0fdfa 100644
--- a/dlls/twain/capability.c
+++ b/dlls/twain/capability.c
@@ -16,6 +16,12 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#define NONAMELESSUNION
+#define NONAMELESSSTRUCT
+
+#include <stdarg.h>
+
+#include "windef.h"
 #include "winbase.h"
 #include "twain.h"
 #include "twain_i.h"