[Patch, Fortran] PR57469 - silence unused dummy arg warning, if it is used in a namelist

[Tobias Burnus]

Tobias Burnus wrote:
> Built and regtested on x86-64-gnu-linux.
> OK for the trunk?

Committed as obvious (Rev. 200785) with the test-case's grammar in the 
comment fixed. (Thanks Dominique for the review.)

Tobias