Runtime Error (atm): vmag_gust not added to pbuf when CLUBB not used

'vmag_gust' is added to pbuf in `clubb_register_cam`. If CLUB is not used, there is a runtime error, as recorded in atm.log:

```
CAM_PIO_CHECK_VAR INFO: variable vmag_gust is not on file
vmag_gust initialized to 1.
ERROR: Attempt to find undefined name in pbuf vmag_gust
```
(As an aside, from the code it looks to me like the default initialized value is 0, not 1.)

This was encountered while merging E3SM/master into ACME-ECP because we do not use CLUBB with superparameterization.

One solution is to move the `pbuf_add_field` call for `vmag_gust` somewhere other than `clubb_register_cam`, but I do not know the best place to move it.