Continue explicitly enabling OpenMP.
Continue explicitly enabling OpenMP.

We used to pass --enable-openmp to the update_and_rebuild_libmesh.sh
script, but --enable-openmp was essentially replaced by
--with-thread-model=openmp in libMesh/libmesh#1579. This commit should
turn that flag back on.

Refs #10740.

nuclear-wizard/moose,bwspenc/moose,nuclear-wizard/moose,nuclear-wizard/moose,YaqiWang/moose,SudiptaBiswas/moose,harterj/moose,jessecarterMOOSE/moose,sapitts/moose,dschwen/moose,idaholab/moose,SudiptaBiswas/moose,andrsd/moose,andrsd/moose,bwspenc/moose,bwspenc/moose,andrsd/moose,permcody/moose,dschwen/moose,milljm/moose,nuclear-wizard/moose,harterj/moose,lindsayad/moose,jessecarterMOOSE/moose,idaholab/moose,laagesen/moose,idaholab/moose,lindsayad/moose,milljm/moose,harterj/moose,permcody/moose,idaholab/moose,laagesen/moose,sapitts/moose,harterj/moose,milljm/moose,bwspenc/moose,milljm/moose,andrsd/moose,YaqiWang/moose,laagesen/moose,sapitts/moose,harterj/moose,dschwen/moose,laagesen/moose,YaqiWang/moose,sapitts/moose,permcody/moose,SudiptaBiswas/moose,bwspenc/moose,sapitts/moose,dschwen/moose,SudiptaBiswas/moose,jessecarterMOOSE/moose,milljm/moose,lindsayad/moose,jessecarterMOOSE/moose,SudiptaBiswas/moose,jessecarterMOOSE/moose,lindsayad/moose,YaqiWang/moose,permcody/moose,dschwen/moose,laagesen/moose,lindsayad/moose,andrsd/moose,idaholab/moose