gsimplecal: Broken on Darwin

Has never had a successful Darwin build.

http://hydra.nixos.org/build/38312308

pkgs/applications/misc/gsimplecal/default.nix

···
     '';
     license = stdenv.lib.licenses.bsd3;
     maintainers = [ stdenv.lib.maintainers.romildo ];
-    platforms = stdenv.lib.platforms.unix;
+    platforms = stdenv.lib.platforms.linux;
   };
 }