libvisual: use gst_element_class_set_metadata when passing dynamic strings
gst_element_class_set_metadata is meant to only be used with static or inlined strings, which isn't the case for this element resulting in use-after-free later on.
gst_element_class_set_metadata is meant to only be used with static or inlined strings, which isn't the case for this element resulting in use-after-free later on.