cairo: always enable XCB support when X backend is enabled

cairo depends on xlib_libX11 for its Xlib backend. But xlib_libX11
depends on XCB, so the XCB support can always be built into Cairo.

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