meson: OpenMP is supposed to be optional

The code around it checks that it is found, so clearly it was meant to
be optional, not arequired.

Fixes: cd2832ee
       ("meson: add an optional OpenMP dependency for AMD tests")

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Tested-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <!10286>