Fix setting method visibility on method wrapped with prepend

Ignore prepended modules when looking for already defined methods on a
class to set the visibility on.
[Fix GH-1834]

From: Dylan Thacker-Smith <Dylan.Smith@shopify.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62725 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_module.rb

@@ -1887,6 +1887,25 @@ class TestModule < Test::Unit::TestCase
     assert_raise(ArgumentError) { Module.new { prepend } }
   end
 
+  def test_prepend_private_super
+    wrapper = Module.new do
+      def wrapped
+        super + 1
+      end
+    end
+
+    klass = Class.new do
+      prepend wrapper
+
+      def wrapped
+        1
+      end
+      private :wrapped
+    end
+
+    assert_equal(2, klass.new.wrapped)
+  end
+
   def test_class_variables
     m = Module.new
     m.class_variable_set(:@@foo, 1)