/usr/include/x86_64-linux-gnu/ is in libqt5serialport5-dev 5.7.1~20161021-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/x86_64-linux-gnu/qt5/
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/QSerialPort
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/QSerialPortInfo
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/QtSerialPort
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/QtSerialPortDepends
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/QtSerialPortVersion
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/qserialport.h
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/qserialportglobal.h
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/qserialportinfo.h
/usr/include/x86_64-linux-gnu/qt5/QtSerialPort/qtserialportversion.h