maint: fix gzip-specific syntax-check rule

maint: fix gzip-specific syntax-check rule

* cfg.mk (sc_gzip_copyright_check): This rule had two problems.
It was failing erroneously (the copyright date in gzip.c is fine),
yet it was complaining.  The first problem is that the rule's
diagnostic mentioned the wrong file: lib/version-etc.c, rather than
./gzip.c. The second problem is that it specified this file name
via "in_files" rather than the "in_vc_files" variable.