FIX build with gfortran11

[t-bltg]

Without this patch, with gfortran 11.1.0:

[...]/src/gotm/gotm.F90:184:56:

  184 |    namelist /model_setup/ title,nlev,dt,restart_offline,restart_allow_missing_variable, &
      |                                                        1
Error: Symbol ‘restart_offline’ in namelist ‘model_setup’ at (1) must be declared before the namelist is declared.
[...]/src/gotm/gotm.F90:184:87:

  184 | _setup/ title,nlev,dt,restart_offline,restart_allow_missing_variable, &
      |                                                                     1

Error: Symbol ‘restart_allow_missing_variable’ in namelist ‘model_setup’ at (1) must be declared before the namelist is declared.
[...]/src/gotm/gotm.F90:185:50:

  185 |                           restart_allow_perpetual,cnpar,buoy_method
      |                                                  1
Error: Symbol ‘restart_allow_perpetual’ in namelist ‘model_setup’ at (1) must be declared before the namelist is declared.

[bolding]

Thanks