RoiDescriptor: Fix linking in dbg build.
A static const class member needs to have a separate definition if it's odr-used, even if it's initialized inline. Here, RoiDescriptor::FULLSCAN is odr-used in HLTSeeding when it is passed to make_unique and hence bound to a reference. Can avoid this by declaring it instead as constexpr. Fixes HLTSeeding link failure in the dbg build.