/usr/mipsel-linux-gnu/lib/librt.so.1 is in libc6-mipsel-cross 2.24-10cross1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/mipsel-linux-gnu/lib/librt-2.24.so
/usr/mipsel-linux-gnu/lib/librt-2.24.so