/usr/include/SILLY/ is in libsilly-dev 0.1.0-8.
This file is owned by root:root, with mode 0o755.
..
/usr/include/SILLY/SILLY.h
/usr/include/SILLY/SILLYBase.h
/usr/include/SILLY/SILLYDataSource.h
/usr/include/SILLY/SILLYDataSource.icpp
/usr/include/SILLY/SILLYFileDataSource.h
/usr/include/SILLY/SILLYFileDataSource.icpp
/usr/include/SILLY/SILLYImage.h
/usr/include/SILLY/SILLYImage.icpp
/usr/include/SILLY/SILLYImageContext.h
/usr/include/SILLY/SILLYImageContext.icpp
/usr/include/SILLY/SILLYImageLoader.h
/usr/include/SILLY/SILLYImageLoader.icpp
/usr/include/SILLY/SILLYImageLoaderManager.h
/usr/include/SILLY/SILLYImageLoaderManager.icpp
/usr/include/SILLY/SILLYMemoryDataSource.h
/usr/include/SILLY/SILLYMemoryDataSource.icpp
/usr/include/SILLY/SILLYOptions.h