swayidle: Pre-emptively fix finding wayland-scanner

Once a new swayidle version is released this will be needed b3f4f9c921

The pc file for `wayland-scanner` was split from `wayland` which causes
the need to `buildBuild` `pkg-config` (in the future) 921bd99.

pkgs/by-name/sw/swayidle/package.nix

@@ -16,6 +16,7 @@ stdenv.mkDerivation rec {
   };
 
   strictDeps = true;
+  depsBuildBuild = [ pkg-config ];
   nativeBuildInputs = [ meson ninja pkg-config scdoc wayland-scanner ];
   buildInputs = [ wayland wayland-protocols ]
                 ++ lib.optionals systemdSupport [ systemd ];