xkbmon: depend on libxcb independently of libx11 propagating it

pkgs/by-name/xk/xkbmon/package.nix

@@ -3,6 +3,7 @@
   stdenv,
   fetchFromGitHub,
   libX11,
+  libxcb,
 }:
 
 stdenv.mkDerivation rec {
@@ -16,7 +17,10 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-EWW6L6NojzXodDOET01LMcQT8/1JIMpOD++MCiM3j1Y=";
   };
 
-  buildInputs = [ libX11 ];
+  buildInputs = [
+    libX11
+    libxcb
+  ];
 
   installPhase = "install -D -t $out/bin xkbmon";