Bug#911996: libqt5serialport5 version in Debian stable is wrong

[Rudolf Polzer]

Package: libqt5serialport5
Version: 5.7.1 is available, 5.9.x is missing
I run Debian stable and need libqt5serialport5 version 5.9 instead of 
5.7 - other parts of Debian stable do not accept qt5 version lower than 5.9