php8: fix xmlreader dom circular dependency

php8: fix xmlreader dom circular dependency

xmlreader was selecting package php8-mod-dom as well as depending on
PHP8_DOM, while php8-mod-dom also depended on PHP8_DOM (and therefore
selected PHP8_DOM when php8-mod-dom was selected). This is a Kconfig
recursive dependency, so break the recursion by noting that because
php8-mod-xmlreader selects php8-mod-dom, PHP8_DOM is a transitive
depends, so php8-mod-xmlreader should not depend on PHP8_DOM itself.

Signed-off-by: Daniel F. Dickinson <dfdpublic@wildtechgarden.ca>