build: Lower libmount dependency to 2.23

Since commit 96ebcee8

, we don’t actually need libmount 2.28. Lower our
dependency to 2.23 so that we can continue to build against CentOS 7.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Reviewed-by: Emmanuele Bassi <ebassi@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=793288