gst-full: add linker flag to keep gst_init_static_plugins

gst-full: add linker flag to keep gst_init_static_plugins

In a fully static link where an app link with gstreamer-full
the gst_init_static_plugins can be discarded because
no one references it.
Indeed the symbol is looked up by gst_init to call if it exists
and so it is not clearly referenced.

In order to avoid this issue, we use the linker flag
--undefined=gst_init_static_plugins to keep
the symbol.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-build/-/merge_requests/207>