x11-wm/mutter: Set libmutter API version as subslot

x11-wm/mutter: Set libmutter API version as subslot

The libraries are now meant to be parallel installable, but we don't split the
libraries from the rest for now to be able to have full parallel installable SLOTs.
So at least express the API version as subslot, so gnome-shell can depend on the
specific one for the libraries. This subslot is only to be used by consumers when
linking to the libmutter libraries (the subslot number equals requested pkg-config API),
mainly gnome-shell.

Package-Manager: Portage-2.3.5, Repoman-2.3.2