Fix problem on Pi caused by unsigned char being default.

Found via #849.

Signed-off-by: Roger A. Light <roger@atchoo.org>

ChangeLog.txt

@@ -14,6 +14,8 @@ Broker:
   just reach it. This was inconsistent in two places.
 - Fix segfault on startup if bridge CA certificates could not be read.
   Closes #851.
+- Fix problem opening listeners on Pi caused by unsigned char being default.
+  Found via #849.
 
 Library:
 - Fix some places where return codes were incorrect, including to the

src/mosquitto_broker_internal.h

@@ -199,7 +199,7 @@ struct mosquitto__security_options {
 	char *acl_file;
 	struct mosquitto__auth_plugin_config *auth_plugin_configs;
 	int auth_plugin_config_count;
-	char allow_anonymous;
+	int8_t allow_anonymous;
 	bool allow_zero_length_clientid;
 	char *auto_id_prefix;
 	int auto_id_prefix_len;