accessibility/at-spi2-core: unbreak build with glib/MANPAGES=off

meson.build:147:13: ERROR: Dependency "libxml-2.0" not found, tried pkgconfig

libxml2 is only used by tests which cannot be disabled. devel/glib20 has it in
LIB_DEPENDS due to USE_GNOME=libxslt:build -> libxslt_USE_GNOME_IMPL=libxml2
without specifying :build suffix.

PR: 274045
Reported by: Anton Saietskii
Submitted by: Olivier Duchateau (based on)

(cherry picked from commit b87eb7985bfeaac9c71e3c01826b585db7c9bad0)