/usr/share/postgresql/9.5/extension/postgis_sfcgal--2.1.2--2.2.1.sql is in postgresql-9.5-postgis-scripts 2.2.1+dfsg-2.
This file is owned by root:root, with mode 0o644.
It is a symlink to /usr/share/postgresql/9.5/extension/./usr/share/postgresql/9.5/extension/postgis_sfcgal--2.1.9--2.2.1.sql