/usr/include/ModemManagerQt/ is in libmodemmanagerqt-dev 1.0.1-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/ModemManagerQt/ModemManagerQt-export.h
/usr/include/ModemManagerQt/bearer.h
/usr/include/ModemManagerQt/generic-types.h
/usr/include/ModemManagerQt/interface.h
/usr/include/ModemManagerQt/manager.h
/usr/include/ModemManagerQt/modem.h
/usr/include/ModemManagerQt/modem3gpp.h
/usr/include/ModemManagerQt/modem3gppussd.h
/usr/include/ModemManagerQt/modemcdma.h
/usr/include/ModemManagerQt/modemdevice.h
/usr/include/ModemManagerQt/modemlocation.h
/usr/include/ModemManagerQt/modemmessaging.h
/usr/include/ModemManagerQt/modemtime.h
/usr/include/ModemManagerQt/sim.h
/usr/include/ModemManagerQt/sms.h