Ensure PETSc headers are included after `bout/petsclib.hxx` #4253

Job	Run time
CUDA	12m 19s
Fedora	18m 50s
CMake, PETSc unreleased, ADIOS2	24m 29s
Default options, Ubuntu 22.04	15m 22s
Optimised, static	15m 21s
Debug, shared	22m 16s
Shared, release	14m 1s
Shared, OpenMP, 3D metrics	14m 9s
CMake, new PETSc	19m 52s
	2h 36m 39s

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