php: fix libxml2 build failure

The libxml2 build test uses the PHP_TEST_BUILD macro which in turn uses
AC_TRY_RUN which is bad for cross compilations.
Force php_cv_libxml_build_works to yes when libxml2 is selected. Fixes:
http://autobuild.buildroot.net/results/8b45a1260ab6ae15dc59a5d6b5b98698ec3e7bbe/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>