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)

accessibility/at-spi2-core/Makefile

@@ -14,7 +14,7 @@ LICENSE_FILE=	${WRKSRC}/COPYING
 PORTSCOUT=	limitw:1,even
 
 USES=		gettext gnome localbase meson pkgconfig tar:xz
-USE_GNOME=	glib20 introspection:build
+USE_GNOME=	glib20 introspection:build libxml2:build
 USE_LDCONFIG=	yes
 MESON_ARGS=	-Duse_systemd=false