util/mkdef.pl, util/add-depends.pl: don't lowercase file names

It turns out to be detrimental on some file systems that may or may not
be case sensitive (such as NTFS, which has a case sensitive mode).

Fixes #7172

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7172)

(cherry picked from commit d3c72e392ab72d418ea5147857701a8730def3fd)