dbus-python: select pyexpat dependency

dbus-python requires xml.parsers.expat, which is provided by the pyexpat
module. This patch enables the pyexpat module as a dependency when dbus-python
is selected.

[Peter: fixup whitespace]
Signed-off-by: Ivan Sergeev <vsergeev@kumunetworks.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>