Fix warning on mingw build.

util.c

@@ -1726,7 +1726,7 @@ bool auth_stratum(struct pool *pool)
 
 static int recv_byte(int sockd)
 {
-	unsigned char c;
+	char c;
 
 	if (recv(sockd, &c, 1, 0) != -1)
 		return c;