Add default systemd feature flag to enable systemd integration

The `libsystemd` crate fails to build on FreeBSD and is presumably
undesirable on any non-systemd distros. At least at present it's easy to
make it optional.

Cargo.toml

@@ -27,7 +27,7 @@ xkbcommon = "0.4"
 indexmap = "1.8.0"
 xdg = "^2.1"
 ron = "0.7"
-libsystemd = "0.5"
+libsystemd = { version = "0.5", optional = true }
 wayland-backend = "0.1.0"
 wayland-scanner = "0.30.0"
 cosmic-protocols = { git = "https://github.com/pop-os/cosmic-protocols", branch = "main", default-features = false, features = ["server"] }
@@ -53,8 +53,9 @@ features = ["svg"]
 optional = true
 
 [features]
-default = []
+default = ["systemd"]
 debug = ["egui", "smithay-egui", "renderdoc", "anyhow/backtrace"]
+systemd = ["libsystemd"]
 
 [profile.dev]
 lto = "thin"