Curl_recv_has_postponed_data: silence compiler warnings

Follow-up to d00f2a8f2
diff --git a/lib/sendf.c b/lib/sendf.c
index c3458a2..7601697 100644
--- a/lib/sendf.c
+++ b/lib/sendf.c
@@ -210,6 +210,8 @@
 /* Use "do-nothing" macros instead of functions when workaround not used */
 bool Curl_recv_has_postponed_data(struct connectdata *conn, int sockindex)
 {
+  (void)conn;
+  (void)sockindex;
   return false;
 }
 #define pre_receive_plain(c,n) do {} WHILE_FALSE