This is a followup to 07ac13a26a, which
omitted to switch to gexps in packages that inherit from 'petsc'.
Reported by Mathieu Othacehe <othacehe@gnu.org>.
* gnu/packages/maths.scm (petsc-complex)[arguments]: Use gexps.
(petsc-openmpi)[arguments]: Likewise.
(petsc-complex-openmpi)[arguments]: Likewise.