ui: fix format specfier in vnc to avoid break in build.

When building qemu after setting _VNC_DEBUG to 1 (see ui/vnc.h),
we get the following error and the build breaks:
...
ui/vnc.c: In function ‘vnc_client_io_error’:
ui/vnc.c:1262:13: error: format ‘%d’ expects argument of type ‘int’, but
             VNC_DEBUG("Closing down client sock: ret %d (%s)\n",
             ^
cc1: all warnings being treated as errors
make: *** [ui/vnc.o] Error 1
...

This patch solves this issue by fixing the print format specifier
in vnc_client_io_error() to be %zd, which corresponds to the type
of the "ret" variable.

Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Message-id: 1484039965-25907-1-git-send-email-rami.rosen@intel.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>