Only call load_iseq if it's defined

In some cases such as loading `netrc` the `super` call is erroring with
`super: no superclass method 'load_iseq' for class
RubyVM::InstructionSequence (NoMethodError)`

Author: KaanOzkan

lib/tapioca/rbs/rewriter.rb

@@ -26,7 +26,7 @@ module ISeq
         module InstructionSequenceMixin
           #: (String) -> RubyVM::InstructionSequence
           def load_iseq(path)
-            super
+            super if defined?(super)
           end
         end
       end