cups: disable php extension

Such an old cups version isn't compatible with newer PHP series, so just
disable it. Fixes:
http://autobuild.buildroot.net/results/2dc/2dc985fc7cff6bd5d20fc074db4f6f7997dd81f5/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>