/usr/lib/x86_64-linux-gnu/libqextserialport.so is in libqextserialport-dev 1.2.0~rc1+git7-g3be3fbf-1+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libqextserialport.so.1.2.0