host-libxml2: enable debug for mesa3d

The API generation script in mesa3d requires libxml2 to be built with
debug, because it uses the lsCountNode function which is only available
in debug mode.

Note that this is the second "temporary" hack to make mesa3d work.
See http://lists.busybox.net/pipermail/buildroot/2012-November/061936.html


for possible more fundamental solutions.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>