mpc-cli: set mainProgram

Signed-off-by: Sefa Eyeoglu <[e-mail]>

pkgs/applications/audio/mpc/default.nix

···
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ AndersonTorres ];
     platforms = with platforms; unix;
+    mainProgram = "mpc";
   };
 }