science/dftbplus: Unbreak build with GCC 9

/usr/local/bin/ld: coulomb.o: in function `coulomb_MOD_invrstress._omp_fn.0':
coulomb.f90:(.text+0x42): undefined reference to `GOMP_loop_maybe_nonmonotonic_runtime_start'
/usr/local/bin/ld: coulomb.f90:(.text+0x176): undefined reference to `GOMP_loop_maybe_nonmonotonic_runtime_next'
/usr/local/bin/ld: coulomb.o: in function `coulomb_MOD_invrstress._omp_fn.1':
coulomb.f90:(.text+0xfbc): undefined reference to `GOMP_loop_maybe_nonmonotonic_runtime_start'
/usr/local/bin/ld: coulomb.f90:(.text+0x1172): undefined reference to `GOMP_loop_maybe_nonmonotonic_runtime_next'
[...]

PR: 238666
Submitted by: jbeich
Reported by: antoine (via exp-run in PR 238330)
Approved by: yuri (maintainer timeout, 2 weeks)