nextpnr: enable OpenMP support (for Eigen)

The new HEaP analytic placer for NextPNR uses Eigen for underlying
placement algorithms. Enabling -DUSE_OPENMP passes -fopenmp onto the
compiler, which Eigen picks up automatically with no extra work.

This should result in placer speedups for large designs, e.g. on ECP5
85k chips.

Signed-off-by: Austin Seipp <aseipp@pobox.com>