libxcb: fix socket writes

The patch has been picked up from libxcb git and is only
applicable to v1.10 while it gets fixed in mainstream v1.11.
http://cgit.freedesktop.org/xcb/libxcb/commit/?id=be0fe56c3bcad5124dcc6c47a2fad01acd16f71a

Signed-off-by: Awais Belal <awais_belal@mentor.com>