Add loop indices to private clauses in 3D viscous GPU loops

Add i,j,k,l to the private list for the 3D shear_stress and
bulk_stress GPU parallel loops, matching the pattern already used
by the analogous 2D loops at lines 105 and 215.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_viscous.fpp

@@ -318,7 +318,7 @@ contains
         #:if not MFC_CASE_OPTIMIZATION or num_dims > 2
 
             if (shear_stress) then    ! Shear stresses
-                $:GPU_PARALLEL_LOOP(collapse=3, private='[rho_visc, gamma_visc, pi_inf_visc, alpha_visc_sum, alpha_visc, alpha_rho_visc, Re_visc, tau_Re]')
+                $:GPU_PARALLEL_LOOP(collapse=3, private='[i,j,k,l,rho_visc, gamma_visc, pi_inf_visc, alpha_visc_sum, alpha_visc, alpha_rho_visc, Re_visc, tau_Re]')
                 do l = is3_viscous%beg, is3_viscous%end
                     do k = -1, 1
                         do j = is1_viscous%beg, is1_viscous%end
@@ -427,7 +427,7 @@ contains
             end if
 
             if (bulk_stress) then    ! Bulk stresses
-                $:GPU_PARALLEL_LOOP(collapse=3, private='[rho_visc, gamma_visc, pi_inf_visc, alpha_visc_sum, alpha_visc, alpha_rho_visc, Re_visc, tau_Re]')
+                $:GPU_PARALLEL_LOOP(collapse=3, private='[i,j,k,l,rho_visc, gamma_visc, pi_inf_visc, alpha_visc_sum, alpha_visc, alpha_rho_visc, Re_visc, tau_Re]')
                 do l = is3_viscous%beg, is3_viscous%end
                     do k = -1, 1
                         do j = is1_viscous%beg, is1_viscous%end