io: Add missing GCC_FMT_ATTR (fix -Werror=suggest-attribute=format)

This fixes a compiler warning:

/qemu/io/channel-websock.c:163:5: error:
 function might be possible candidate for ‘gnu_printf’ format attribute
 [-Werror=suggest-attribute=format]

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Acked-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

io/channel-websock.c

@@ -151,9 +151,10 @@ enum {
     QIO_CHANNEL_WEBSOCK_OPCODE_PONG = 0xA
 };
 
-static void qio_channel_websock_handshake_send_res(QIOChannelWebsock *ioc,
-                                                   const char *resmsg,
-                                                   ...)
+static void GCC_FMT_ATTR(2, 3)
+qio_channel_websock_handshake_send_res(QIOChannelWebsock *ioc,
+                                       const char *resmsg,
+                                       ...)
 {
     va_list vargs;
     char *response;