gst-vocoder.sh: use vocoder_1337.so from swh-plugins

The copy in lmms is not available for other programs to use.

After I filed https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=758888
it was made available in /usr/lib/ladspa:
https://salsa.debian.org/debian-edu-pkg-team/lmms/commit/754c10e#58ef006ab62b83b4bec5d81fe5b32c3b4c2d1cc2

But then it was hidden again because the one in swh-plugins became
available:
https://salsa.debian.org/debian-edu-pkg-team/lmms/commit/a58ea7c7c719df57ce51ba163faf6603b87d81cb