default-crate-overrides: add libseat-sys

This is needed for building `cosmic-comp` using `crate2nix` instead
of `buildRustPackage` (like
https://github.com/NixOS/nixpkgs/pull/251365/ does).

pkgs/build-support/rust/default-crate-overrides.nix

@@ -40,6 +40,7 @@
 , autoconf
 , automake
 , libtool
+, seatd # =libseat
 , ...
 }:
 
@@ -172,6 +173,11 @@ in
     buildInputs = [ openssl zlib libgit2 ];
   };
 
+  libseat-sys = attrs: {
+    nativeBuildInputs = [ pkg-config ];
+    buildInputs = [ seatd ];
+  };
+
   libsqlite3-sys = attrs: {
     nativeBuildInputs = [ pkg-config ];
     buildInputs = [ sqlite ];