Small fix for Windows compilation

Hi!

Could you fix small problem for Windows compilation (stable/master branches)?

diff --git a/anon.c b/anon.c
index 083d5bd..af6b669 100644
--- a/anon.c
+++ b/anon.c
@@ -24,7 +24,7 @@ PG_MODULE_MAGIC;
 #ifdef _WIN64
 PGDLLEXPORT void    _PG_init(void);
 PGDLLEXPORT Datum   get_function_schema(PG_FUNCTION_ARGS);
-PGDLLEXPORT void    register_label(PG_FUNCTION_ARGS);
+PGDLLEXPORT Datum   register_label(PG_FUNCTION_ARGS);
 #else
 void    _PG_init(void);
 Datum   get_function_schema(PG_FUNCTION_ARGS);