flat_namespace_allowlist: add `hdf5-mpi`
flat_namespace_allowlist: add `hdf5-mpi`

The `-flat_namespace` flag is inherited from `open-mpi`. See
`ompi-fort.pc`, which contains:

    Cflags: -I${includedir}     -Wl,-flat_namespace -Wl,-commons,use_dylibs  -I${libdir}

This is needed for bottling on Monterey.

uyjulian/homebrew-core,zyedidia/homebrew-core,nbari/homebrew-core,zyedidia/homebrew-core,nandub/homebrew-core,chrisfinazzo/homebrew-core,filcab/homebrew-core,makigumo/homebrew-core,nbari/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,lemzwerg/homebrew-core,mfikes/homebrew-core,nandub/homebrew-core,adam-moss/homebrew-core,lemzwerg/homebrew-core,adam-moss/homebrew-core,chrisfinazzo/homebrew-core,Homebrew/homebrew-core,makigumo/homebrew-core,filcab/homebrew-core,sjackman/homebrew-core,cblecker/homebrew-core,sjackman/homebrew-core,stgraber/homebrew-core,mfikes/homebrew-core,cblecker/homebrew-core,Homebrew/homebrew-core