Dependency fix. We observed that doing any change to class THD in sql_class.h resulted in mysqld going crazy (parsing errors,
query cache errors in query_cache.test). This is because sql_yacc.cc depends on several .h files but those were not listed in
the dependencies of sql_yacc.o. The present patch does fix the issue; but my auto*-expert colleagues may have a better one.


sql/Makefile.am:
  sql_yacc.c includes mysql_priv.h, slave.h etc, so sql_yacc.o does depend on these .h files;
  so they need to be listed in the dependencies.