bpo-32604: Make _xxsubinterpreters build on Windows (GH-5624)

This is not the ideal solution; this means that a test module is now
always included in the main python3x.dll.  However, we're already
including xxsubtype, so why not?
(cherry picked from commit 310b05289b5d9550040f469e60b5e8e77f1022b6)

Co-authored-by: Zachary Ware <zachary.ware@gmail.com>
diff --git a/PC/config.c b/PC/config.c
index 6209d7e..0c08bfe 100644
--- a/PC/config.c
+++ b/PC/config.c
@@ -31,7 +31,9 @@
 #endif
 extern PyObject* PyInit__codecs(void);
 extern PyObject* PyInit__weakref(void);
+/* XXX: These two should really be extracted to standalone extensions. */
 extern PyObject* PyInit_xxsubtype(void);
+extern PyObject* PyInit__xxsubinterpreters(void);
 extern PyObject* PyInit_zipimport(void);
 extern PyObject* PyInit__random(void);
 extern PyObject* PyInit_itertools(void);
@@ -126,6 +128,7 @@
     {"_json", PyInit__json},
 
     {"xxsubtype", PyInit_xxsubtype},
+    {"_xxsubinterpreters", PyInit__xxsubinterpreters},
     {"zipimport", PyInit_zipimport},
 #ifdef _Py_HAVE_ZLIB
     {"zlib", PyInit_zlib},