[cmake] ilmbase: add 2 possible input folders
[cmake] ilmbase: add 2 possible input folders

Alembic uses direct includes to ilmbase (half.h) while AliceVision use includes with the project prefix (OpenEXR/half.h), so here we add both folders.
poparteu/openMVG,poparteu/openMVG,poparteu/openMVG,poparteu/openMVG