After this change 6df3e602a7 (diff-24324a084e511502d7f34054ec31b353L50), an explicit include for KdTree is needed.
It was formerly a transitive include via pcl/surface/mls.h - pcl/search/pcl_search.h - pcl/search/kdtree.h - pcl/kdtree/kdtree_flann.h.