fix: add flux_tau to GPU private list in s_lf_riemann_solver

type(riemann_states) structs are not auto-private under OpenACC (unlike
plain real(wp) scalars). flux_tau_L/flux_tau_R were scalars on master and
worked without explicit listing; after the riemann_states refactor they
became flux_tau (a derived type) which requires explicit private=[flux_tau].
HLLC had it; LF was missed.

Author: sbryngelson

src/simulation/m_riemann_solvers.fpp

@@ -876,7 +876,7 @@ contains
                                     & B2, b4, cm, pcorr, zcoef, vel_grad_L, vel_grad_R, idx_right_phys, vel_rms, vel_avg_rms, &
                                     & vel_tmp, Ms, pres_S, alpha_L_sum, alpha_R_sum, c_avg, pres, rho, gamma, pi_inf, qv, c, E, &
                                     & H, ptilde, s_M, s_P, xi_M, xi_P, Cp_avg, Cv_avg, T_avg, eps, c_sum_Yi_Phi, Cp, Cv, R_gas, &
-                                    & MW, T, Y]')
+                                    & MW, T, Y, flux_tau]')
                 do l = ${Z_BND}$%beg, ${Z_BND}$%end
                     do k = ${Y_BND}$%beg, ${Y_BND}$%end
                         do j = ${X_BND}$%beg, ${X_BND}$%end