Fix MPI variable in utilities.cc from int to unsigned int.

[MFraters]

resolves #770. @anne-glerum, could you confirm this solves the issue for you?

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.211 ± 0.011 (s=382)	1.212 ± 0.011 (s=363)	-0.1% .. +0.3%
Slab interpolation curved simple none	1.210 ± 0.008 (s=383)	1.211 ± 0.014 (s=363)	-0.1% .. +0.4%
Spherical slab interpolation simple none	1.224 ± 0.009 (s=345)	1.226 ± 0.011 (s=392)	-0.1% .. +0.3%
Slab interpolation simple curved CMS	1.253 ± 0.011 (s=353)	1.253 ± 0.012 (s=368)	-0.2% .. +0.2%
Spherical slab interpolation simple CMS	1.639 ± 0.014 (s=280)	1.640 ± 0.015 (s=272)	-0.1% .. +0.3%
Spherical fault interpolation simple none	1.231 ± 0.010 (s=382)	1.232 ± 0.011 (s=351)	-0.1% .. +0.3%
Cartesian min max surface	2.731 ± 0.028 (s=145)	2.731 ± 0.022 (s=187)	-0.4% .. +0.4%
Spherical min max surface	7.745 ± 0.049 (s=54)	7.741 ± 0.069 (s=65)	-0.5% .. +0.4%

[anne-glerum]

Confirmed! This fixes the compiler warnings on my machine.

[MFraters]

Thanks for confirming!