Skip refined method when exporting methods with changed visibility

	Previously, attempting to change the visibility of a method in a
	singleton class for a class/module that is prepended to and refined
	would raise a NoMethodError.

	Fixes [Bug #17519]
	---
	 test/ruby/test_module.rb | 23 +++++++++++++++++++++++
	 vm_method.c              | 14 +++++++++++---
	 2 files changed, 34 insertions(+), 3 deletions(-)