Fix warning on mingw build.
diff --git a/util.c b/util.c
index 550f4ed..52ab46b 100644
--- a/util.c
+++ b/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;