/usr/include/allegro5/platform/ is in liballegro5-dev 2:5.0.10-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/allegro5/platform/albcc32.h
/usr/include/allegro5/platform/aliphone.h
/usr/include/allegro5/platform/aliphonecfg.h
/usr/include/allegro5/platform/almngw32.h
/usr/include/allegro5/platform/almsvc.h
/usr/include/allegro5/platform/alosx.h
/usr/include/allegro5/platform/alosxcfg.h
/usr/include/allegro5/platform/alucfg.h
/usr/include/allegro5/platform/alunix.h
/usr/include/allegro5/platform/alwatcom.h
/usr/include/allegro5/platform/alwin.h
/usr/include/allegro5/platform/astdbool.h
/usr/include/allegro5/platform/astdint.h