Two "make distcheck" fixes for Fortran

1. Ensure to clean up generated module files (make distcheck complains if there are files left in the build tree).
1. Don't distribute sizeof_f.f90; it's generated (this bug was masked on master, but was just fixed in open-mpi/ompi@9d1d34c0c050dbef136874b97d4df04ab0029c51)