BUILD: ot: use a cebtree instead of a list for variable names

BUILD: ot: use a cebtree instead of a list for variable names

In order for the function flt_ot_vars_scope_dump() to work, it is
necessary to take into account the changes made by the commits 47ec7c681
("OPTIM: vars: use a cebtree instead of a list for variable names") and
5d350d1e5 ("OPTIM: vars: use multiple name heads in the vars struct").

The function is only used if the OT_DEBUG=1 option is set when compiling
HAProxy.