meson: Only require libexpat when a part of the build needs it.

Now that xmlconfig can be built without libexpat on Android, we can make
android builds not require the presence of libexpat for many drivers.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7223>
2 files changed