tinyasm - use PetscMPIInt not PetscInt for PetscSFGetRootRanks

[JHopeCollins]

Description

If building with 64 bit integers, compiling tinyasm results in the following error:

    tinyasm/tinyasm.cpp:184:47: error: cannot convert ‘PetscInt*’ {aka ‘long int*’} to ‘PetscMPIInt*’ {aka ‘int*’}
      184 |             ierr = PetscSFGetRootRanks(sf[i], &nranks, &ranks, NULL, NULL, NULL);CHKERRQ(ierr);
          |                                               ^~~~~~~
          |                                               |
          |                                               PetscInt* {aka long int*}

PetscSFGetRootRanks has the following signature (https://petsc.org/release/manualpages/PetscSF/PetscSFGetRootRanks/):

PetscSFGetRootRanks(PetscSF sf, PetscMPIInt *nranks, const PetscMPIInt **ranks, ...)

but nranks is declared as PetscInt.

This PR changes the declaration of nranks to PetscMPIInt. The j variable is also changed to PetscMPIInt - this is fine because the only thing j is used for is a loop index with maximum extent nranks.

[connorjward]

This is obviously right.

[github-actions[bot]]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake complex	8068 ran	6487 passed	1581 skipped	0 failed

[github-actions[bot]]

	Tests	Passed ✅	Skipped ⏭️	Failed ❌
Firedrake real	8074 ran	7288 passed	786 skipped	0 failed