win32: update Fedora version for mingw build

Version is changed from 25 to 27 as the former one is entering EOL
soon.

Change-Id: I38eb0c687101a45ecd24531694e41118295c5fd1
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

docker/Dockerfile_windows_mingw

-FROM fedora:25
+FROM fedora:27
 
 ENV LANG en_US.utf8
 ENV LC_ALL en_US.utf8