profiles: fix missing call to force:aux_enable()

For the stress profile, the stress tensor needs to be computed in a
sampling step. The error occurs if the sampling intervals of
thermodynamics and profile differ from each other.

The issue is reproduced as follows (since thermodynamics is sampled
every 100 steps by default).

$ halmd -vvv --time=10 profiles --every=1

    [TRACE] performing MD step #2
    [TRACE] compute maximum squared displacement
    [TRACE] profiles: acquire sample
    [TRACE] profiles: compute profile bins
    [TRACE] profiles: sort bin IDs
    [TRACE] profiles: find boundaries
    [TRACE] profiles: collect stress tensors
    [ERROR] Auxiliary variables were not enabled in force module.
    [WARNING] HALMD aborted