cairo: add missing dependency on xlib_libXext

The X.org backend of Cairo now requires xlib_libXext. See
http://permalink.gmane.org/gmane.comp.lib.cairo/23385.

Fixes:

 http://autobuild.buildroot.org/results/795404665ec74a7ac929fdf8caf1397c9c54d80d/build-end.log



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