nixos/kde-pim: include kcontacts for merkuro's widgets

When using Merkuro in Plasma 6, the 'Contacts' widget will automatically
be added to the default panel. Unfortunately, it does not work unless
`kdePackages.kcontacts` is also present in the environment. This adds
the dependency if Plasma 6 is also enabled alongside Merkuro.

Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>

nixos/modules/programs/kde-pim.nix

@@ -33,8 +33,14 @@ in
       ++ lib.optionals cfg.kontact [
         kontact
       ]
-      ++ lib.optionals cfg.merkuro [
-        merkuro
-      ];
+      ++ lib.optionals cfg.merkuro (
+        [
+          merkuro
+        ]
+        # Only needed when using the Merkuro Contacts widget in Plasma.
+        ++ lib.optionals config.services.desktopManager.plasma6.enable [
+          kcontacts
+        ]
+      );
   };
 }