Ensure PETSc headers are included after `bout/petsclib.hxx` #4254

Job	Run time
CUDA	12m 12s
Fedora	18m 23s
CMake, PETSc unreleased, ADIOS2	23m 6s
Default options, Ubuntu 22.04	14m 4s
Optimised, static	15m 37s
Debug, shared	21m 54s
Shared, release	15m 0s
Shared, OpenMP, 3D metrics	15m 6s
CMake, new PETSc	15m 6s
Coverage	14m 13s
	2h 44m 41s

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