[ruby/pp] Only do RubyVM patches if class exists

This class does not exist in any implementation except CRuby.

I would recommend moving this code somewhere else, like a separate
file loaded only on CRuby or into CRuby itself. For now this
change is sufficient to load the library on other implementations.

https://github.com/ruby/pp/commit/7d5a220f64

lib/pp.rb

@@ -539,6 +539,7 @@ class MatchData # :nodoc:
   end
 end
 
+if defined?(RubyVM::AbstractSyntaxTree)
   class RubyVM::AbstractSyntaxTree::Node
     def pretty_print_children(q, names = [])
       children.zip(names) do |c, n|
@@ -572,6 +573,7 @@ class RubyVM::AbstractSyntaxTree::Node
       }
     end
   end
+end
 
 class Object < BasicObject # :nodoc:
   include PP::ObjectMixin