vnc: Fix compile error on x86_64 with -D_VNC_DEBUG=1

vnc: Fix compile error on x86_64 with -D_VNC_DEBUG=1

cc1: warnings being treated as errors
vnc-auth-sasl.c: In function ‘vnc_client_write_sasl’:
vnc-auth-sasl.c:50: error: format ‘%d’ expects type ‘int’, but argument 4 has type ‘size_t’
vnc-auth-sasl.c:50: error: format ‘%d’ expects type ‘int’, but argument 5 has type ‘size_t’
make: *** [vnc-auth-sasl.o] Error 1

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>