package/libxml2: fix libxml2-config.cmake

This patch aligns the libxml2-config.cmake module to what the libxml-2.0.pc set.

This patch has already been sent upstream:
  https://mail.gnome.org/archives/xml/2014-December/msg00015.html



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>