games-engines/frotz: Add missing sound dependencies

games-engines/frotz: Add missing sound dependencies

libvorbis isn't actually used directly, apart from in the Makefile,
where it's included in the link, presumably for static
linking. Support in libsndfile or libsdl2 is expected to be present
though so I have included it anyway.

I haven't included minimum versions as stated in INSTALL because it
admits that older versions may work and we generally don't bother
unless older versions are still in the tree or were until recently.

Closes: https://bugs.gentoo.org/698434
Package-Manager: Portage-2.3.78, Repoman-2.3.17
Signed-off-by: James Le Cuirot <chewi@gentoo.org>