knownnetworks: fix printing SSID in hex

Force conversion to unsigned char before printing to avoid sign
extension when printing SSID in hex. For example, if there are CJK
characters in SSID, it will generate a very long string like
/net/connman/iwd/ffffffe8ffffffaeffffffa1.

src/knownnetworks.c

@@ -174,7 +174,7 @@ static const char *known_network_get_path(const struct network_info *network)
 
 	for (i = 0; network->ssid[i] && pos < sizeof(path); i++)
 		pos += snprintf(path + pos, sizeof(path) - pos, "%02x",
-				network->ssid[i]);
+				(unsigned char)network->ssid[i]);
 
 	if (pos < sizeof(path))
 		snprintf(path + pos, sizeof(path) - pos, "_%s",