mx_mysql: Add a unused_result warning

All callers were made to handle errors returned by
mx_mysql_option_set_default_file() in thee previou commit. Now add the
function attribute `warn_unused_result` to it, so that future callers
will do, too.

mx_mysql.h

@@ -77,7 +77,7 @@ struct mx_mysql_stmt {
 
 int mx_mysql_initialize(struct mx_mysql **mysql);
 
-int mx_mysql_option_set_default_file(struct mx_mysql *mysql, char *fname);
+int mx_mysql_option_set_default_file(struct mx_mysql *mysql, char *fname)  __attribute__ ((warn_unused_result));
 int mx_mysql_option_set_default_group(struct mx_mysql *mysql, char *group);
 int mx_mysql_option_set_reconnect(struct mx_mysql *mysql, int reconnect);