/usr/share/boinc-dev/api/reduce.h is in libboinc-app-dev 7.6.31+dfsg-6.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/boinc-dev/api/../../../include/boinc/api/reduce.h
/usr/share/boinc-dev/api/../../../include/boinc/api/reduce.h