meson: add rpath on the modules_install_dir for all modules

Otherwise the dynamic loader won't be able to find libraries in modules_install_dir we link against

This MR just goes ahead and slaps RPATH on all modules, but as of right now it's only required for libpipewire-module-client-node.so. The RPATH on the other libs doesn't hurt though and prevents this situation from occurring again with another lib.

fixes #214 (closed)