bluetuith: add meta.mainProgram

Signed-off-by: Paul Meyer <[e-mail]>

pkgs/tools/bluetooth/bluetuith/default.nix

···
     homepage = "https://github.com/darkhz/bluetuith";
     license = licenses.mit;
     platforms = platforms.linux;
+    mainProgram = "bluetuith";
     maintainers = with maintainers; [ thehedgeh0g ];
   };
 }