upb: use the new reflection headers

Reflection headers now live in upb/reflection/ so update our include statements to
reflect this. (See what I did there?)

PiperOrigin-RevId: 474596615
diff --git a/python/descriptor_pool.c b/python/descriptor_pool.c
index ea0e813..4e9faaa 100644
--- a/python/descriptor_pool.c
+++ b/python/descriptor_pool.c
@@ -32,7 +32,7 @@
 #include "python/descriptor.h"
 #include "python/message.h"
 #include "python/protobuf.h"
-#include "upb/def.h"
+#include "upb/reflection/def.h"
 #include "upb/util/def_to_proto.h"
 
 // -----------------------------------------------------------------------------