FindMPI: Place static first in mpi test source

Move the `static` added by commit 8cce0ad3 (FindMPI: Fix missing
static warning in the mpi test source, 2021-06-30, v3.22.0-rc1~502^2) to
the beginning of the line.

Fixes: #23141
36 jobs for !6900 with FindMPI-static-first in 12 minutes and 53 seconds (queued for 4 seconds)
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