Fixing openmp compilation issue

[nikizadehgfdl]

• New code does not compile with -openmp : ice_model.F90(979): error #6752: Since the OpenMP* DEFAULT(NONE) clause applies, the PRIVATE, SHARED, REDUCTION, FIRSTPRIVATE, or LASTPRIVATE attribute must be explicitly specified for every variable. [US]
• This PR fixes the issue by adding US as shared

[marshallward]

Gaea regression: https://gitlab.gfdl.noaa.gov/ogrp/SIS2/-/pipelines/17593 :heavy_check_mark:

[marshallward]

Thank you Niki. We should figureout some way to include OpenMP testing of SIS2.