package/php: drop --with-libxml-dir

--with-libxml-dir has been dropped since version 7.4.0 and
https://github.com/php/php-src/commit/29d1b7fd521af288e9f04d784f59a2d15b494a30



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>