include serial_mesh.h in PatternedMesh.h
include serial_mesh.h in PatternedMesh.h

Against typical libMesh builds that happens indirectly via mesh.h via
MooseMesh.h, but for libMesh --enable-parmesh builds we need to
include the SerialMesh definition explicitly.

This fixes #6957 for me.

liuwenf/moose,liuwenf/moose,bwspenc/moose,dschwen/moose,idaholab/moose,jessecarterMOOSE/moose,laagesen/moose,jessecarterMOOSE/moose,andrsd/moose,nuclear-wizard/moose,SudiptaBiswas/moose,Chuban/moose,jessecarterMOOSE/moose,jasondhales/moose,milljm/moose,SudiptaBiswas/moose,yipenggao/moose,harterj/moose,nuclear-wizard/moose,permcody/moose,backmari/moose,sapitts/moose,andrsd/moose,idaholab/moose,permcody/moose,milljm/moose,backmari/moose,friedmud/moose,Chuban/moose,joshua-cogliati-inl/moose,lindsayad/moose,bwspenc/moose,nuclear-wizard/moose,dschwen/moose,permcody/moose,laagesen/moose,idaholab/moose,milljm/moose,liuwenf/moose,harterj/moose,laagesen/moose,dschwen/moose,jasondhales/moose,idaholab/moose,lindsayad/moose,andrsd/moose,milljm/moose,backmari/moose,permcody/moose,friedmud/moose,Chuban/moose,YaqiWang/moose,YaqiWang/moose,joshua-cogliati-inl/moose,dschwen/moose,friedmud/moose,sapitts/moose,nuclear-wizard/moose,SudiptaBiswas/moose,lindsayad/moose,SudiptaBiswas/moose,harterj/moose,dschwen/moose,stimpsonsg/moose,katyhuff/moose,sapitts/moose,YaqiWang/moose,bwspenc/moose,stimpsonsg/moose,milljm/moose,jasondhales/moose,jhbradley/moose,stimpsonsg/moose,katyhuff/moose,jessecarterMOOSE/moose,jasondhales/moose,SudiptaBiswas/moose,yipenggao/moose,jhbradley/moose,YaqiWang/moose,liuwenf/moose,katyhuff/moose,jessecarterMOOSE/moose,idaholab/moose,liuwenf/moose,andrsd/moose,joshua-cogliati-inl/moose,laagesen/moose,harterj/moose,sapitts/moose,friedmud/moose,jhbradley/moose,backmari/moose,Chuban/moose,bwspenc/moose,jhbradley/moose,katyhuff/moose,sapitts/moose,lindsayad/moose,harterj/moose,yipenggao/moose,lindsayad/moose,bwspenc/moose,joshua-cogliati-inl/moose,andrsd/moose,laagesen/moose,stimpsonsg/moose,yipenggao/moose,liuwenf/moose