Fix concurrency bug in "make test_headers"
Fix concurrency bug in "make test_headers"

I noticed this was broken while swiping the code for TIMPI in
https://github.com/libMesh/TIMPI/pull/37 - we should fix it here too.

roystgnr/libmesh,BalticPinguin/libmesh,dschwen/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,roystgnr/libmesh,roystgnr/libmesh,roystgnr/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,dschwen/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,jwpeterson/libmesh,BalticPinguin/libmesh,dschwen/libmesh,roystgnr/libmesh,libMesh/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,jwpeterson/libmesh,libMesh/libmesh,jwpeterson/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,dschwen/libmesh,libMesh/libmesh,dschwen/libmesh,libMesh/libmesh,BalticPinguin/libmesh