clean reduce windows build warnings

Signed-off-by: Andy Green <andy.green@linaro.org>
diff --git a/lib/private-libwebsockets.h b/lib/private-libwebsockets.h
index 203377a..0e67aa2 100644
--- a/lib/private-libwebsockets.h
+++ b/lib/private-libwebsockets.h
@@ -628,7 +628,7 @@
  */
 
 struct allocated_headers {
-	unsigned short next_frag_index;
+	unsigned char next_frag_index;
 	unsigned short pos;
 	unsigned char frag_index[WSI_TOKEN_COUNT];
 	struct lws_fragments frags[WSI_TOKEN_COUNT * 2];