Fix a bug in the OMP directive for plume_flux

  Changed a recently added OMP directive for plume_flux from private to
firstprivate to reflect how this variable is actually used.  This bug was
introduced with PR #401, but was causing sporadic failures in some of our
pipeline tests with the intel compiler (essentially due to initialized
memory when openMP is used) for subsequent commits.

src/parameterizations/vertical/MOM_diabatic_aux.F90

@@ -1207,8 +1207,8 @@ subroutine applyBoundaryFluxesInOut(CS, G, GV, US, dt, fluxes, optics, nsw, h, t
   !$OMP                                  drhodt,drhods,pen_sw_bnd_rate,                    &
   !$OMP                                  pen_TKE_2d,Temp_in,Salin_in,RivermixConst,        &
   !$OMP                                  mixing_depth,A_brine,fraction_left_brine,         &
-  !$OMP                                  plume_flux,plume_fraction,dK)                     &
-  !$OMP                     firstprivate(SurfPressure)
+  !$OMP                                  plume_fraction,dK)                     &
+  !$OMP                     firstprivate(SurfPressure,plume_flux)
   do j=js,je
   ! Work in vertical slices for efficiency