declare a development shell in flake.nix (#372)

This makes it possible to build i3lock (for development)
on any system on which Nix can be installed (= most Linux systems).

For example, I start Emacs using `nix develop --command emacs`
and that Emacs process is in an environment with all i3lock build deps.

See also:
https://michael.stapelberg.ch/posts/2025-07-27-dev-shells-with-nix-4-quick-examples/

This file is provided best-effort, but PRs are welcome.

Author: stapelberg

flake.lock

@@ -0,0 +1,28 @@
+{
+  description = "development environment for i3lock";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.05";
+  };
+
+  outputs = { self, nixpkgs }:
+    let
+      system = "x86_64-linux";
+      pkgs = nixpkgs.legacyPackages.${system};
+    in {
+      devShells.${system}.default = pkgs.mkShell {
+        buildInputs = with pkgs; [
+          cairo
+          libev
+          libxkbcommon
+          pam
+          pkg-config
+          xcbutilxrm
+          xorg.libX11
+          xorg.libxcb
+          xorg.xcbutil
+          xorg.xcbutilimage
+        ];
+      };
+    };
+}