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.

https://gstreamer.freedesktop.org/documentation/gstreamer/gstelement.html?gi-language=c#gst_element_class_set_static_metadata