/usr/include/allegro/platform/ is in liballegro4-dev 2:4.4.2-4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/allegro/platform/aintbeos.h
/usr/include/allegro/platform/aintdos.h
/usr/include/allegro/platform/aintlnx.h
/usr/include/allegro/platform/aintmac.h
/usr/include/allegro/platform/aintosx.h
/usr/include/allegro/platform/aintpsp.h
/usr/include/allegro/platform/aintqnx.h
/usr/include/allegro/platform/aintunix.h
/usr/include/allegro/platform/aintwin.h
/usr/include/allegro/platform/al386gcc.h
/usr/include/allegro/platform/al386vc.h
/usr/include/allegro/platform/al386wat.h
/usr/include/allegro/platform/albcc32.h
/usr/include/allegro/platform/albecfg.h
/usr/include/allegro/platform/albeos.h
/usr/include/allegro/platform/aldjgpp.h
/usr/include/allegro/platform/aldmc.h
/usr/include/allegro/platform/aldos.h
/usr/include/allegro/platform/almac.h
/usr/include/allegro/platform/almaccfg.h
/usr/include/allegro/platform/almngw32.h
/usr/include/allegro/platform/almsvc.h
/usr/include/allegro/platform/alosx.h
/usr/include/allegro/platform/alosxcfg.h
/usr/include/allegro/platform/alplatf.h
/usr/include/allegro/platform/alpsp.h
/usr/include/allegro/platform/alpspcfg.h
/usr/include/allegro/platform/alqnx.h
/usr/include/allegro/platform/alqnxcfg.h
/usr/include/allegro/platform/alucfg.h
/usr/include/allegro/platform/alunix.h
/usr/include/allegro/platform/alunixac.h
/usr/include/allegro/platform/alwatcom.h
/usr/include/allegro/platform/alwin.h
/usr/include/allegro/platform/astdint.h
/usr/include/allegro/platform/macdef.h