php: fix WDDX extension build failure

Fix WDDX extension build failure.
The documentation and source are conflicting about this, in theory
libexpat support is deprecated and libxml2 can replace it, but it seems
not so for WDDX, so pull in expat for it.

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