Link the gnome-shell binary to libgnome-shell-js.so

Depending on the exact linker options and versions, rpath's written
into the binary may, or may not, be honored by dlopen(). Work around
this by simply linking the gnome-shell binary to gnome-shell-js, since
then dlopen() doesn't need to search paths.

https://bugzilla.gnome.org/show_bug.cgi?id=670477