Excise DL_EXPORT from Include.

Thanks to Skip Montanaro and Kalle Svensson for the patches.
diff --git a/Include/enumobject.h b/Include/enumobject.h
index df20fb0..053fb72 100644
--- a/Include/enumobject.h
+++ b/Include/enumobject.h
@@ -7,7 +7,7 @@
 extern "C" {
 #endif
 
-extern DL_IMPORT(PyTypeObject) PyEnum_Type;
+PyAPI_DATA(PyTypeObject) PyEnum_Type;
 
 #ifdef __cplusplus
 }