jamulus: set meta.mainProgram

pkgs/applications/audio/jamulus/default.nix

···
     homepage = "https://github.com/corrados/jamulus/wiki";
     license = lib.licenses.gpl2Plus;
     platforms = lib.platforms.linux;
+    mainProgram = "jamulus";
     maintainers = [ lib.maintainers.seb314 ];
   };
 }