win64: make openssl work

fix windows style path separator to unix style, needed for cygwin.

Change-Id: I4de78d6901378644857c28a59467b59ef886f47b
Reviewed-on: https://gerrit.libreoffice.org/10855
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>