swaylock-effects: fix pkg-config of wayland-scanner

The split out of wayland-scanner from wayland into its own package:
921bd99
caused issues with pkg-config not finding wayland-scanner anymore.
This commit fixes this by adding pkg-config to depsBuildBuild as done
for other packages having the same issues like:
ff49827

pkgs/by-name/sw/swaylock-effects/package.nix

@@ -30,6 +30,7 @@ stdenv.mkDerivation rec {
   '';
 
   strictDeps = true;
+  depsBuildBuild = [ pkg-config ];
   nativeBuildInputs = [ meson ninja pkg-config scdoc wayland-scanner];
   buildInputs = [ wayland wayland-protocols libxkbcommon cairo gdk-pixbuf pam ];