ice_dyn_vp: activate OpenMP in 'dyn_prep2' loop

When the OpenMP implementation was reviewed and fixed in d1e972a (Update
OMP (CICE-Consortium#680), 2022-02-18), the 'PRIVATE' clause of the OpenMP directive
for the loop where 'dyn_prep2' is called in 'implicit_solver' was
corrected in line with what was done in 'ice_dyn_evp', but OpenMP was
left unactivated for this loop (the 'TCXOMP' was not changed to a real
'OMP' directive).

Activate OpenMP for this loop. All runs and restart tests of the
'decomp_suite' still pass with this change.

cicecore/cicedynB/dynamics/ice_dyn_vp.F90

@@ -341,9 +341,7 @@ subroutine implicit_solver (dt)
          call grid_average_X2Y('F',strairyT,'T',strairyU,'U')
       endif
 
-! tcraig, tcx, threading here leads to some non-reproducbile results and failures in icepack_ice_strength
-! need to do more debugging
-      !$TCXOMP PARALLEL DO PRIVATE(iblk,ilo,ihi,jlo,jhi,this_block,ij,i,j)
+      !$OMP PARALLEL DO PRIVATE(iblk,ilo,ihi,jlo,jhi,this_block,ij,i,j)
       do iblk = 1, nblocks
 
       !-----------------------------------------------------------------
@@ -411,7 +409,7 @@ subroutine implicit_solver (dt)
          enddo  ! ij
 
       enddo  ! iblk
-      !$TCXOMP END PARALLEL DO
+      !$OMP END PARALLEL DO
 
       call icepack_warnings_flush(nu_diag)
       if (icepack_warnings_aborted()) call abort_ice(error_message=subname, &