export hx509_ca_tbs_set_signature_algorithm

hx509_ca_tbs_set_signature_algorithm was added by commit
c69c4634ad2296b003d50f3a4180298c6f37d30f.  It must be exported
for use by hxtool on Windows.

Change-Id: I14b927abde96814ae2e0a90f232ab00915a9f29e