Install pkg-copyright.txt as pkg-licenses.txt

* src/inst-gpg4win.nsi, src/uninst-gpg4win.nsi: Install / Uninstall
it.
* src/make-msi.pl: Add as a fixed component.

Since a list of all included copyright holders is nearly impossible to
obtain I would rather call the file licenses as it contains the licenses
of all the packages.

• GnuPG-Bug-Id: T6834