/usr/share/boinc-dev/lib/diagnostics.h is in libboinc-app-dev 7.2.42+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/boinc-dev/lib/../../../include/boinc/diagnostics.h
/usr/share/boinc-dev/lib/../../../include/boinc/diagnostics.h