cmake: use the correct file name for the Perl header

cmake: use the correct file name for the Perl header

In e4b488049a5 (Makefile: extract script to massage Perl scripts,
2024-12-06), the code was refactored that is used to transform the Perl
scripts/modules to their final form.

Even the CMake-based build was adjusted, but the change used the file
name `PERL-HEADER` instead of the file name used by the Makefile-based
build (same name but with the `GIT-` prefix). Let's adjust the former to
the latter.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>