internals/finish: do not remove ${PREFIX}/include

gcc installs the C++ headers in ${PREFIX}/include/ but we trash
that directory at the end of the build.

We previously removed that directory as it contained the companion
libraries header files. But it's been some time now that we isntall
the companion libraries in their own dedicated place, so we do not
need to remove that directory.

Until we have a better fix, just keep that directory for now.

Reported-by: Bob Rossi <bob@brasko.net>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

COPYING