/usr/include/tcl8.6/tcl-private/ is in tcl8.6-dev 8.6.5+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/tcl8.6/tcl-private/compat/
/usr/include/tcl8.6/tcl-private/compat/dirent.h
/usr/include/tcl8.6/tcl-private/compat/dirent2.h
/usr/include/tcl8.6/tcl-private/compat/dlfcn.h
/usr/include/tcl8.6/tcl-private/compat/fake-rfc2553.h
/usr/include/tcl8.6/tcl-private/compat/float.h
/usr/include/tcl8.6/tcl-private/compat/stdlib.h
/usr/include/tcl8.6/tcl-private/compat/string.h
/usr/include/tcl8.6/tcl-private/compat/unistd.h
/usr/include/tcl8.6/tcl-private/generic/
/usr/include/tcl8.6/tcl-private/generic/regcustom.h
/usr/include/tcl8.6/tcl-private/generic/regerrs.h
/usr/include/tcl8.6/tcl-private/generic/regex.h
/usr/include/tcl8.6/tcl-private/generic/regguts.h
/usr/include/tcl8.6/tcl-private/generic/tcl.h
/usr/include/tcl8.6/tcl-private/generic/tclCompile.h
/usr/include/tcl8.6/tcl-private/generic/tclDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclFileSystem.h
/usr/include/tcl8.6/tcl-private/generic/tclIO.h
/usr/include/tcl8.6/tcl-private/generic/tclInt.h
/usr/include/tcl8.6/tcl-private/generic/tclIntDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclIntPlatDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclOO.h
/usr/include/tcl8.6/tcl-private/generic/tclOODecls.h
/usr/include/tcl8.6/tcl-private/generic/tclOOInt.h
/usr/include/tcl8.6/tcl-private/generic/tclOOIntDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclParse.h
/usr/include/tcl8.6/tcl-private/generic/tclPlatDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclPort.h
/usr/include/tcl8.6/tcl-private/generic/tclRegexp.h
/usr/include/tcl8.6/tcl-private/generic/tclStringTrim.h
/usr/include/tcl8.6/tcl-private/generic/tclTomMath.h
/usr/include/tcl8.6/tcl-private/generic/tclTomMathDecls.h
/usr/include/tcl8.6/tcl-private/generic/tclTomMathInt.h
/usr/include/tcl8.6/tcl-private/generic/tommath.h
/usr/include/tcl8.6/tcl-private/libtommath/
/usr/include/tcl8.6/tcl-private/libtommath/tommath.h
/usr/include/tcl8.6/tcl-private/libtommath/tommath_class.h
/usr/include/tcl8.6/tcl-private/libtommath/tommath_superclass.h
/usr/include/tcl8.6/tcl-private/unix/
/usr/include/tcl8.6/tcl-private/unix/tclUnixPort.h
/usr/include/tcl8.6/tcl-private/unix/tclUnixThrd.h