pkgsMusl.dbus: 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 D-Bus disabled by default for pkgsMusl.

pkgs/development/libraries/dbus/default.nix

···
 , fetchurl
 , pkg-config
 , expat
-, enableSystemd ? stdenv.isLinux && !stdenv.hostPlatform.isMusl
+, enableSystemd ? stdenv.isLinux && !stdenv.hostPlatform.isStatic
 , systemd
 , audit
 , libapparmor