cairo: do not depend on xserver-xorg_server, but on the right library

cairo is a X client library, so there is no reason for it to build
depend on the X.org server. What Cairo needs is the xlib_libX11
library.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>