Store FFI::Type::Mapped of FFI::StrPtrConverter in global instead of …

…custom type map

The custom type map was prefilled like so before:
{FFI::StrPtrConverter=>#<FFI::Type::Mapped::0x00005587914f66f0 size=8 alignment=8>}

lib/ffi/types.rb

@@ -60,7 +60,7 @@ def self.add_typedef(old, add)
 
   class << self
     private def __typedef(old, add)
-      TypeDefs[add] = self.find_type(old)
+      TypeDefs[add] = self.find_type(old, TypeDefs)
     end
 
     private :custom_typedefs