Fix st ANYARGS warnings

The use of ANYARGS by st.h causes warnings such as the following:

avro/lang/c/src/st.c:240:13: warning: passing arguments to a function without a prototype is deprecated in all versions of C and is not
        supported in C2x [-Wdeprecated-non-prototype]
          hash_val = do_hash(key, table);

The use of ANYARGS can be replaced with void* and appropriate casts