dbus: do not build XML DOCS

Setting the variable DBUS_ENABLE_XML_DOCS to OFF will not build this
documentation.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>

utils/dbus/Makefile

@@ -88,7 +88,8 @@ CMAKE_OPTIONS += \
 	-DDBUS_DISABLE_CHECKS=ON \
 	-DDBUS_BUILD_X11=OFF \
 	-DDBUS_ENABLE_DOXYGEN_DOCS=OFF \
-	-DDBUS_ENABLE_QTHELP_DOCS=OFF
+	-DDBUS_ENABLE_QTHELP_DOCS=OFF \
+	-DDBUS_ENABLE_XML_DOCS=OFF
 
 define Build/InstallDev
 	$(INSTALL_DIR) $(1)/usr/include