php: force cross-compilation

This fixes build problems like
http://autobuild.buildroot.org/results/e3c5815766fdcf3f13b06fa0758ec418618ac4be/build-end.log

.

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