tpm2-util: properly load tpm2 libraries befre unsealing
    
    We forgot a call to dlopen_tpm2() in the unseal codepaths. As long as
    automatic TPM2 device discovery was used that didn't matter, since in
    that codepaths we'd have another call dlopen_tpm2(). But with an
    explicitly configured TPM2 device things should work too, hence add the
    missing call.
    
    Fixes: #19206
    (cherry picked from commit 1b30720c1b22df75e147cc201f240db23881672d)