mu: no relation to mesa so set platforms directly

For reasons unknown, the list of support platforms tracked that of mesa.
That makes no sense whatsoever, so set it to a list of platforms that
actually should be supported.

pkgs/tools/networking/mu/default.nix

@@ -50,6 +50,6 @@ stdenv.mkDerivation rec {
     homepage = "https://www.djcbsoftware.nl/code/mu/";
     changelog = "https://github.com/djcb/mu/releases/tag/v${version}";
     maintainers = with maintainers; [ antono chvp peterhoeg ];
-    platforms = platforms.mesaPlatforms;
+    platforms = platforms.unix;
   };
 }