Workaround for incompatibility between vectorclass 1 and 2

the next version of LCG will feature vectorclass 2 (https://sft.its.cern.ch/jira/browse/SPI-1619), which happens to have an API change that causes failures in lcg-dev slots.

This should fix the problem.

There's no need to backport this to run2-patches, because the situation is much worse there and some specific work is needed.