s390/net: add Thorsten Winkler as maintainer

Thank you Wenjia for your support, welcome Thorsten!

Acked-by: Wenjia Zhang <wenjia@linux.ibm.com>
Acked-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>