Fix duplicate error values.

Closes #1109. Thanks to Tifaifai.

lib/mosquitto.h

@@ -89,10 +89,10 @@ enum mosq_err_t {
 	MOSQ_ERR_MALFORMED_UTF8 = 18,
 	MOSQ_ERR_KEEPALIVE = 19,
 	MOSQ_ERR_LOOKUP = 20,
-	MOSQ_ERR_MALFORMED_PACKET = 19,
-	MOSQ_ERR_DUPLICATE_PROPERTY = 20,
-	MOSQ_ERR_TLS_HANDSHAKE = 21,
-	MOSQ_ERR_QOS_NOT_SUPPORTED = 22,
+	MOSQ_ERR_MALFORMED_PACKET = 21,
+	MOSQ_ERR_DUPLICATE_PROPERTY = 22,
+	MOSQ_ERR_TLS_HANDSHAKE = 23,
+	MOSQ_ERR_QOS_NOT_SUPPORTED = 24,
 };
 
 /* Error values */