dwm(picom): Don't show shadow on top of bar

Fixes an issue where there would previously be two shadows at the top
with a window open without having to exclude shadow from the bar.

specializations/dwm.nix

@@ -116,6 +116,10 @@ in
             method = "dual_kawase";
             size = 10;
           };
+
+          clip-shadow-above = [
+            "class_g = 'dwm'"
+          ];
         };
       };