Teach Windows build about the _weakref module.
diff --git a/PC/config.c b/PC/config.c
index c96eb26..2044688 100644
--- a/PC/config.c
+++ b/PC/config.c
@@ -43,6 +43,7 @@
 #endif
 extern void init_codecs(void);
 extern void initxreadlines(void);
+extern void init_weakref(void);
 
 /* XXX tim: what's the purpose of ADDMODULE MARKER? */
 /* -- ADDMODULE MARKER 1 -- */
@@ -95,6 +96,7 @@
 
         {"_codecs", init_codecs},
 	{"xreadlines", initxreadlines},
+	{"_weakref", init_weakref},
 
 /* XXX tim: what's the purpose of ADDMODULE MARKER? */
 /* -- ADDMODULE MARKER 2 -- */