wire generator: reverse subtype struct order

nested subtypes need to come first in the struct declaration. we
reverse the order that we read them off so that this requirement
is met

tools/generate-wire.py

@@ -1355,7 +1355,7 @@ def build_tlv_structs(tlv_fields):
 
 def build_subtype_structs(subtypes):
     structs = ""
-    for subtype in subtypes:
+    for subtype in reversed(subtypes):
         structs += subtype.print_struct()
     return structs