meson: Install libgnomekbd{,ui}.so.8

Previously, the meson build system would install:

 - libgnomekbd.so.3.28.0 with symlinks from
   - libgnomekbd.so.8.0.0
   - libgnomekbd.so

This didn't match the autotools build system, which installed:

 - libgnomekbd.so.8.0.0 with symlinks from
   - libgnomekbd.so.8
   - libgnomekbd.so

The ABI didn't change between v3.26.1 (which only had autotools) and
v3.28.0, so I don't think we should install .so.3.28.0 shared objects.

This change makes meson install the same shared objects and symlinks as
autotools.