Fix falsely deleted nml variable (!104)

## What is the bug
The variable `nnml` is falsely deleted in !94

. Which causes error in dace that uses `nnml` from `mo_namelist`.
## How do you fix it
Add back the variable.

Merged-by: Yen-Chen Chen <yen-chen.chen@kit.edu>
Changelog: bugfix

src/mo_namelist.F90

@@ -37,6 +37,7 @@ MODULE mo_namelist
 
   PUBLIC :: open_nml_output
   PUBLIC :: close_nml_output
+  PUBLIC :: nnml
 
   !  return values of function 'position_nml':