Log Message:
Some programs don't look in libxml/headername.h, and even libxml itself
does not create the directory (any longer?).
Be a bit defensive about this and add -I${BUILDLINK_DIR}/include/libxml
to BUILDLINK_CONFIG_WRAPPER_SED, while letting -I${BUILDLINK_DIR}/include
stay, too.

XXX: This should be revisited, and the packages depending on libxml2 be fixed
(either way); for now, this change fixes libwmf, ogle, and ogle_gui (and
perhaps others), while not changing the situation for kdelibs2 (which
I didn't want to test because it's too big).

Per a discussion with Johnny Lam.