docs: Add qemu-storage-daemon(1) manpage to meson.build

docs: Add qemu-storage-daemon(1) manpage to meson.build

In commit 1982e1602d15 we added a new qemu-storage-daemon(1) manpage.
At the moment new manpages have to be listed both in the conf.py for
Sphinx and also in docs/meson.build for Meson. We forgot the second
of those -- correct the omission.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20210108161416.21129-2-peter.maydell@linaro.org