[MPI] fix violation of one-definition-rule in GuardCommunicator
GuardCommunicator::create(MPI_Comm)
violated the one-definition-rule. This leads to linker errors, when working with multiple compilation units.
We work around this problem by marking the method inline. We could also put it into the library and only keep the declaration in the header, but this function is so tiny that we actually want to have it inlined.
This should be backported to 2.7