Add missing public interface for t_char_workaround (!106)

## What is the bug
The `t_char_workaround` was missing from the public interface, which causes some BuildBots to fail.
## How do you fix it
Add `t_char_workaround` public interface to `fortran_support`.

Approved by: Pradipta Samanta <samanta@dkrz.de>
Merged-by: Pradipta Samanta <samanta@dkrz.de>
Changelog: default