Extend the unknown-attributes warning workaround to ROOT 6.24

In !2912 (merged) @chasse added a protection against a warning from cling for ROOT 6.22, but the problem is still present in ROOT 2.64 (LCG 100).

assigned to @ascarabo

We can merge this, right @clemenci?

Yes

Assigning @jonrob

assigned to @jonrob and unassigned @ascarabo

mentioned in issue Moore#273 (closed)

merged

mentioned in commit 35ce0992

unassigned @jonrob

mentioned in issue Moore#278 (closed)

mentioned in issue Moore#291 (closed)