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>
1 file changed