opaques: don't set/import/export the size of "Opaques", it is always zero

opaques entries in a tlb have no size. not sure here, if this would
break stuff elsewhere... how to check that this property is not
read/modified anywhere? at least an "amake" inside "base/orogen/types"
with this commit included still works. no runtime-testing.

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>