pkgsMusl.cups: enable systemd support

We previously weren't able to build systemd for Musl, but now we
can! (But not statically.) So there's no longer any reason to have
systemd support in CUPS disabled by default for pkgsMusl.

pkgs/misc/cups/default.nix

···
 , libtiff
 , pam
 , dbus
-, enableSystemd ? stdenv.isLinux && !stdenv.hostPlatform.isMusl
+, enableSystemd ? stdenv.isLinux
 , systemd
 , acl
 , gmp