move the extension registry down into mini_table/

Now that :minitable depends on :hash, we may as well put the extreg there also.

PiperOrigin-RevId: 490620348
diff --git a/upb/fuzz_test_util.h b/upb/fuzz_test_util.h
index 10728fd..4e1e968 100644
--- a/upb/fuzz_test_util.h
+++ b/upb/fuzz_test_util.h
@@ -31,7 +31,7 @@
 #include <string>
 #include <vector>
 
-#include "upb/extension_registry.h"
+#include "upb/mini_table/extension_registry.h"
 #include "upb/mini_table/types.h"
 
 namespace upb {