interDyMFoam: support relaxation of p_rgh

applications/solvers/multiphase/interFoam/interDyMFoam/pEqn.H

@@ -54,6 +54,8 @@
         {
             phi = phiHbyA - p_rghEqn.flux();
 
+            p_rgh.relax();
+
             U = HbyA + rAU*fvc::reconstruct((phig - p_rghEqn.flux())/rAUf);
             U.correctBoundaryConditions();
             fvOptions.correct(U);