Plugin installed directory to <prefix>/lib/, not to GST specific plugin subdirectory

[eero-t]

Use-case:

• build / install gstreamer-media-SDK with Meson

Actual outcome:

• libgstmfx.so* GST plugin goes to {install-prefix}/lib/

Expected outcome:

• GST plugin(s) go to {install-prefix}/lib/gstreamer-{apiversion}/

If plugin goes to same place where all the other libraries are, GST needs to scan & try all of them. And if it does that every time given directory contents change, most of those operations / GST plugin cache rebuilding will happen unnecessarily.

This seems to be a regression from #54.