package/systemd: needs any UTF-8 locale

Not all distributions have the language-agnostic C.UTF-8 locale (Gentoo,
I'm frowning at you!).

Instead, use any UTF-8 locale provided by the system.

Reported-by: Christian Stewart <kidovate@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Julius Kriukas <julius@kriukas.lt>
Cc: Christian Stewart <kidovate@gmail.com>
Cc: Trent Piepho <tpiepho@impinj.com>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>