Mark 'comm' As Potentially Unused

The communicator is only needed in parallel builds.

opm/simulators/utils/readDeck.cpp

@@ -218,7 +218,7 @@ namespace {
     }
 
     std::shared_ptr<Opm::EclipseState>
-    createEclipseState(Opm::Parallel::Communication comm,
+    createEclipseState([[maybe_unused]] Opm::Parallel::Communication comm,
                        const Opm::Deck&                              deck)
     {
 #if HAVE_MPI