php7-mod-xmlreader: add conditional dependency to php7-mod-dom (fixes #10201)

php7-mod-xmlreader: add conditional dependency to php7-mod-dom (fixes #10201)

PHP7 fails to load xmlreader.so (php7-mod-xmlreader) module without
dom.so (php7-mod-dom) module loaded:

-snip-
PHP Warning:  PHP Startup: Unable to load dynamic library 'xmlreader.so'
 (tried: /usr/lib/php/xmlreader.so (Error relocating /usr/lib/php/xmlreader.so:
 dom_node_class_entry: symbol not found), /usr/lib/php/xmlreader.so.so (Error
 loading shared library /usr/lib/php/xmlreader.so.so: No such file or
 directory)) in Unknown on line 0
^C
-snap-

However, this dependency only exists when during build also php7-mod-dom
is selected.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>