gst-full: add linker flag to keep gst_init_static_plugins
requested to merge dabrain34/gst-build:dab_fix_undefined_gst_init_static_plugins_upstream into master
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.